INDUSTRIAL  CHEMISTRY 

BEING  A  SERIES  OP  VOLUMES  fllVING 
A  COMPREHENSIVE  SURVEY  OF 

THE  CHEMICAIi  INDUSTRIES  J 


FRANKLIN  INSTITUTE 
LIBRARY 


LEVYTYPE  CO,  PHI  LA 


FRANKLIN  INSTITUTE  LIBRARY 

PHILADELPHIA 


Class  .  6  .6  . f.-iS  Book.  W.  S-^  Accession 


INDUSTRIAL  CHEMISTRY 


BEING  A  SERIES  OF  VOLUMES  GIVING  A 
COMPREHENSIVE  SURVEY  OF 

THE  CHEMICAL  INDUSTRIES 

Edited  by  SAMUEL  RIDEAL,  D.Sc.  Lond.,  F.I.C. 

FELLOW  OF  UNIVERSITY  COLLEGE,  LONDON 


ASSISTED  BY 


JAMES  A.  AUDLEY,  B.Sc,  F.I.C. 
W.  BACON,  B.Sc,  F.I.C,  F.C.S. 

E.  DE  BARRY  BARNETT,  B.Sc,  A.I.C. 
M.  BARROWCLIFF,  F.I.C. 

H.  GARNER  BENNETT,  M.Sc. 

F.  H.  CARR,  F.I.C. 

S.  HOARE  COLLINS,  M.Sc,  F.I.C. 
H.  C.  GREENWOOD,  O.B.E.,  D.Sc, 
F.I.C. 

&c., 


R.  S.  MORRELL,  M.A.,  Ph.D.,  F.I.C 
J.  R.  PARTINGTON,  M.A.,  Ph.D. 
ARTHUR  E.  PRATT,  B.Sc, Assoc R.S.M. 
ERIC  K.  RIDEAL,  M.A.,  Ph.D.,  F.I.C. 
W.  H.  SIMP^IONS,  B.Sc,  F.I.C. 
R.  W.  SINDALL,  F.C.S. 
HUGH  S.  TAYLOR,  D.Sc. 
A.  de  WAELE,  A.I.C. 
C.  M.  WHITTAKER.  B.Sc 
&c. 


Digitized  by  the  Internet  Arcliive 
in  2015 


littps://arcliive.org/details/rubberresinspainOOmorr 


RUBBER,  RESINS,  PAINTS 
AND  VARNISHES 


R.  S.  MORRELL,  M.A.  (Cantab.),  Ph.D. 

F.I.C. 

AND 

A.  de  WAELE,  A.I.C. 


/ 


NEW  YORK 
D.   VAN    NOSTRAND  COMPANY 

EIGHT  WARREN  STREET 
1920 


Of  7 


1 


PRINTED  IN  GREAT  BRITAIN 


GENERAL  PREFACE 


The  rapid  development  of  Applied  Chemistry  in  recent  years 
has  brought  about  a  revolution  in  all  branches  of  technology. 
This  growth  has  been  accelerated  during  the  war,  and  the 
British  Empire  has  now  an  opportunity  of  increasing  its 
industrial  output  by  the  application  of  this  knowledge  to  the 
raw  materials  available  in  the  different  parts  of  the  world. 
The  subject  in  this  series  of  handbooks  will  be  treated  from 
the  chemical  rather  than  the  engineering  standpoint.  The 
industrial  aspect  will  also  be  more  prominent  than  that  of 
the  laboratory.  Each  volume  will  be  complete  in  itself,  and 
will  give  a  general  survey  of  the  industry,  showing  how 
chemical  principles  have  been  applied  and  have  affected 
manufacture.  The  influence  of  new  inventions  on  the 
development  of  the  industry  will  be  shown,  as  also  the 
effect  of  industrial  requirements  in  stimulating  invention. 
Historical  notes  will  be  a  feature  in  dealing  with  the 
different  branches  of  the  subject,  but  they  will  be  kept 
within  moderate  limits  Present  tendencies  and  possible 
future  developments  will  have  attention,  and  some  space 
will  be  devoted  to  a  comparison  of  industrial  methods  and 
progress  in  the  chief  producing  cotmtries.  There  will  be  a 
general  bibliography,  and  also  a  select  bibliography  to  follow 
each  section.  Statistical  information  will  only  be  introduced 
in  so  far  as  it  serves  to  illustrate  the  line  of  argument. 

Each  book  will  be  divided  into  sections  instead  of 
chapters,  and  the  sections  will  deal  with  separate  branches 
of  the  subject  in  the  manner  of  a  special  article  or  mono- 
graph.   An  attempt  will,  in  fact,  be  made  to  get  away  from 


vi 


GENERAL  PREFACE 


the  orthodox  textbook  manner,  not  only  to  make  the  treat- 
ment original,  but  also  to  appeal  to  the  very  large  class  of 
readers  already  possessing  good  textbooks,  of  which  there 
are  quite  sufficient.  The  books  should  also  be  found  useful 
by  men  of  affairs  having  no  special  technical  knowledge,  but 
who  may  require  from  time  to  time  to  refer  to  technical 
matters  in  a  book  of  moderate  compass,  with  references  to 
the  large  standard  works  for  fuller  details  on  special  points 
if  required. 

To  the  advanced  student  the  books  should  be  especially 
valuable.  His  mind  is  often  crammed  with  the  hard  facts 
and  details  of  his  subject  which  crowd  out  the  power  of 
realizing  the  industry  as  a  whole.  These  books  are  intended 
to  remedy  such  a  state  of  affairs.  While  recapitulating  the 
essential  basic  facts,  they  will  aim  at  presenting  the  reality 
of  the  living  industry.  It  has  long  been  a  drawback  of  our 
technical  education  that  the  college  graduate,  on  commencing 
his  industrial  career,  is  positively  handicapped  by  his 
academic  knowledge  because  of  his  lack  of  information  on 
current  industrial  conditions.  A  book  giving  a  compre- 
hensive survey  of  the  industry  can  be  of  very  material 
assistance  to  the  student  as  an  adjunct  to  his  ordinary  text- 
books, and  this  is  one  of  the  chief  objects  of  the  present 
series.  Those  actually  engaged  in  the  industry  who  have 
specialized  in  rather  narrow  limits  will  probably  find  these 
books  more  readable  than  the  larger  textbooks  when  they 
wish  to  refresh  their  memories  in  regard  to  branches  of  the 
subject  with  which  they  are  not  immediately  concerned. 

The  volume  will  also  serve  as  a  guide  to  the  standard 
literature  of  the  subject,  and  prove  of  value  to  the  con- 
sultant, so  that,  having  obtained  a  comprehensive  view  of 
the  whole  industry,  he  can  go  at  once  to  the  proper 
authorities  for  more  elaborate  information  on  special  points, 
and  thus  save  a  couple  of  days  spent  in  hunting  through  the 
libraries  of  scientific  societies. 

As  far  as  this  country  is  concerned,  it  is  believed  that 
the  general  scheme  of  this  series  of  handbooks  is  unique, 
and  it  is  confidently  hoped  that  it  will  supply  mental 


GENERAL  PREFACE 


vii 


munitions  for  the  coming  industrial  war.  I  have  been 
fortunate  in  securing  writers  for  the  different  volumes  who 
are  specially  connected  with  the  several  departments  of 
Industrial  Chemistry,  and  trust  that  the  whole  series  will 
contribute  to  the  further  development  of  applied  chemistry 
throughout  the  Empire. 

SAMUEL  RIDEAU 


AUTHORS'  PREFACE 

It  has  been  our  endeavour  to  present  to  the  advanced 
student  a  brief  summary  of  the  properties  of  paints  and 
varnishes  together  with  those  of  their  components,  and  a 
general  statement  of  the  principles  underlying  their  manu- 
facture. It  is  of  importance  to  lay  stress  on  the  urgent 
need  for  research  in  the  domain  of  oils  and  resins,  not  only 
on  their  chemical  properties  and  composition  but  on  those 
properties  due  to  surface  action  and  catalysis.  Many 
properties  of  solutions  of  suspensoids  and  emulsoids  are 
presented  in  varnishes  and  paints  which  are  only  recently 
receiving  systematic  investigation.  Paints  and  varnishes 
have  long  been  considered  solely  from  the  craftsman/s  stand- 
point. Progress  has  been  uneven,  and  from  the  methods  of 
investigation  employed  the  industry  is  wrapped  in  a  thick 
cloak  of  trade  secrets. 

In  the  description  of  the  manufacture  of  varnishes  and 
paints  care  has  been  taken  to  avoid  technical  details  beyond 
what  are  required  to  illustrate  general  principles.  In  dealing 
with  paints  the  essential  requirements  are  set  forth  without 
burdening  the  student  with  details  of  formulae  which  are 
often  untrustworthy.  We  have  attempted  to  treat  the 
subject  in  the  spirit  of  the  Editor's  General  Preface  and 
are  indebted  to  him  for  the  section  on  the  Rubber 
Hydrocarbons. 

A  section  on  the  linoleum  industry  has  been  included, 
for  the  reason  that  linoleum,  paints  and  varnishes,  have 
much  in  common  from  the  standpoint  of  the  general 
properties  of  drying  oils. 

We  desire  to  express  our  thanks  to  Messrs.  Mander 
Brothers,  Wolverhampton,  and  to  Mr.  D.  Gestetner  of  the 
Neo- Cyclostyle  Works,  Tottenham  Hale,  lyondon,  for  per- 
mission to  collaborate  in  the  writing  of  this  book. 

ix 


X 


AUTHORS'  PREFACE 


We  are  glad  to  acknowledge  the  assistance  given]  by 
Messrs.  Constable  &  Co.,  Ivtd.,  Messrs.  Manlove,  AUiott  &  Co., 
Messrs.  Rose,  Down  and  Thompson,  and  Messrs.  Torrance 
&  Sons,  Ltd.,  in  the  representation  of  plant  used  in  the 
industries. 

Our  thanks  are  also  due  to  Mr.  P.  J.  Fay,  M.A.,  for  reading 
the  proof  sheets  while  the  work  was  passing  through  the 
press. 

R.  S.  M. 
A.  de  W. 

Wolverhampton  and  London. 
December^  1920. 


CONTENTS 


PART  I. 

THE  RUBBER  HYDROCARBONS 

(INDIARUBBER  :  CAOUTCHOUC). 

PAGE 

Introduction.  The  raw  materials,  their  formation  in  nature,  their  distri- 
bution. Statistics.  Plantation  Rubber.  Collection  of  latex  and 
coagulation.  Tapping.  Composition  of  Hevea  rubber  from  trees  of 
different  ages.  Tapping.  Basal  v.  half  herring.  Daily  and  alternate 
tapping.  Bark  stripping.  Effects  of  tapping.  Wound  response. 
Physical  and  chemical  properties  of  latex.  Coagulation,  with  acid, 
other  chemical  reagents.  The  theory  of  coagulation.  Purification. 
Drying.  Smoking.  Chemical  and  physical  properties  and  testing  of 
rubber.  Manufacture  of  rubber  articles.  Masticating,  mixing, 
vulcanization.  Rubber  substitutes.  Ebonite.  Seeds  and  their  oils. 
Diseases  and  pests.    Costs.    Synthetic  rubber.    Bibliography    •       •  i 


PART  II. 

DRYING  OILS. 

Linseed  oil  as  a  characteristic  drying  oil  in  paints,  linoleum,  and  varnishes. 
Oxidation  of  linseed  oil.  Methods  of  extraction  of  linseed  oil.  Perilla 
oil.  Soya  Bean  oil.  Pararubber  seed  oil.  Poppy  seed  oil.  China 
wood  oil.  Candlenut  oil,  and  other  vegetable  drying  oils.  Boiled, 
blown,  and  stand  oils.  Theories  of  driers.  Methods  of  testing  the 
commoner  drying  oils    .........  35 


PART  III. 

RESINS  AND  PITCHES. 

Varnish  resins.  Formation  of  resins  in  the  plant.  Classification  of  resins^ 
Oil  varnish  resins.  Copals.  Acidity  of  copals.  Spirit  varnish  resins. 
Turpentine.  Turpentine  substitutes.  Colophony,  rosin  oil.  Chinese 
and  Japanese  lacquer.  Synthetic  resins.  Accroides  resin.  Pitches. 
Coal-tar  pitch.  Petroleum  pitches.  Asphaltum.  Stearine  pitches. 
Rosin  pitch.    The  examination  of  pitches  77 

xt 


xii 


CONTENTS 


PART  IV. 

Section  I.— PIGMENTS  AND  PAINTS. 

PAGE 

General  properties  of  pigments.  Colour,  opacity,  and  tinctorial  power. 
Chemical  effect  on  the  medium.  Physical  effect  upon  the  paint.  The 
white  pigments,  manufacture  and  properties.  Fillers  and  extenders. 
Yellow  and  orange  pigments.  Red  pigments.  Red  lakes.  Brown 
pigments.  Purple,  blue,  green,  and  black  pigments.  The  requirements 
of  a  paint  or  enamel  and  its  function  as  a  protective  coating.  Manufacture 
of  paints  and  enamels.  Commercial  paints.  Distemper  and  water 
paints  io8 

Section    II.— LINOLEUM,  CORK  CARPET,  AND 
FLOORCLOTH. 

Linoleum.  Scrim  *'  process >  *'  Shower  bath"  process.  Walton  "  Smacker." 
Cementing  process.  Walton  inlaid  linoleum.  Cork  carpet.  Cor- 
ticine "  process.    Floorcloth  .        .        .        .        .        .        .  .163 


PART  V. 

VARNISHES. 
Section  I.— OIL  VARNISHES. 

Oil  varnishes  :  the  general  properties  of  a  resin,  oil  and  thinners-mixing. 
Manufacture  of  oil  varnishes.  Varnishes  from  tung  oil.  The  properties 
of  varnish  on  application.    Defects  of  varnish.    Types  of  varnishes. 


Japans  and  bituminous  varnishes.    .        .        .        .        .        .  .178 

Section  II.— INSULATING  VARNISHES. 

Varnishes  for  impregnating  windings,  papers,  and  fabrics.     Varnishes  for 

cementing  and  finishing  purposes.    Bakelite  insulating  varnishes    .        .  206 

Section  III.— SPIRIT  VARNISHES. 

Preparation.    Shellac,  manila  and  acaroides  spirit  varnishes      .       .  .213 

Section  IV.— CELLULOSE  ESTER  VARNISHES. 

Collodion.    Celluloid.    Cellulose  acetate  varnishes.       Dopes"       .       .  219 

Section  V.— ANALYSIS  OF  VARNISHES  .  222 

,       •  ■ — ■  

BIBLIOGRAPHY. 

Part  II. — Drying  Oils   229 

Part  III. — Resins  and  Pitches   229 

Part  IV. — Pigments,  Paints,  and  Linoleum  .....  230 

Part  V.— Varnishes   230 

Subject  Index  231 

Name  Index  234 


RUBBER,   RESINS,  PAINTS 
AND  VARNISHES 


Part  I.— THE  RUBBER  HYDROCARBONS 

(Indiarubber  :  Caoutchouc) 

The  British  rubber  plantation  industry  has  reached  its 
present  position  more  by  luck  and  enterprise  than  systematic 
cultivation  trials  :  and,  as  in  many  other  industries,  the 
initial  success  was  brought  about  without  much  scientific 
thought.  By  this,  we  do  not  mean  that  the  pioneer  work  of 
men  like  Hancock,  who  suggested  cultivating  rubber  in  the 
East  and  West  Indies  early  in  the  last  century,  Collins  in 
Singapore  sixty  years  ago,  and  the  later  success  of  Wickham. 
in  Ceylon  with  Hevea  Braziliensis,  have  not  more  than 
justified  the  boldness  with  which  these  earlier  enterprises 
were  marked.  It  is  rather  to  express  surprise  that  so  much 
has  been  done  at  our  botanical  experimental  stations  by 
men  like  Ridley  at  Singapore,  and  the  Indian  Government 
at  Heneratgoda  and  Peradeniya  in  Ceylon,  with  the  small 
amount  of  scientific  assistance  and  monetary  help  placed 
at  the  disposal  of  planters  in  these  two  typical  British 
tropical  centres. 

Nearly  all  the  effort  of  the  last  thirty  years  has  been 
directed  to  the  trial  and  selection  of  native  and  wild  rubbers 
in  different  climates  and  soils  rather  than  any  attempt  at 
botanical  experimental  cultivation  for  producing  new 
varieties  with  increased  yield  of  latex  or  greater  resisting 
power  to  infectious  disease.  Nothing  comparable  to  the 
Canadian  improvement  in  wheats  or  the  phenomenal 
development  in  the  cultivation  of  the  sugar  beet  on  the 
s.  I 


2    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Continent  has  at  present  been  achieved  in  the  case  of 
cultivated  rubber.  The  transplantation  and  cultivation 
of  the  cinchona  in  relation  to  the  yield  of  quinine  shows  the 
possibilities  in  this  direction. 

vSeeds  from  Hevea  trees,  derived  from  Malay,  which 
have  sprung  up  from  the  parent  stock  in  Ceylon,  have  now 
been  grown  in  almost  all  known  likely  rubber-producing 
areas  in  the  tropical  world,  so  that  data  are  now  available 
for  a  careful  study  of  the  conditions  which  contribute  to 
economic  growth.  The  Cicely  estate  at  Teluk  Anson  has 
sent  seeds  and  cuttings  to  both  East  and  West  Africa  and 
to  districts  so  far  remote  as  Fiji  and  Queensland,  and  it 
would  seem  probable  that  the  older  plantations  of  Ceara 
on  the  East  and  Funtumia  on  the  West  coast  of  Africa  will 
give  place  to  the  Hevea  variety,  and  that  consequently  the 
breeding  of  this  rubber  will  probably  ensure  its  survival  as 
the  fittest  for  industrial  growth. 

For  improving  the  yield,  and  at  the  same  time  ensuring 
a  robust  and  long-lived  tree,  the  necessity  for  long  period 
trials  from  selected  seeds  is  essential ;  development  of  our 
knowledge  must  necessarily  be  slower  than  in  the  case  of 
annuals  like  wheat  or  sugar  beet.  Our  present  supremacy 
in  this  plantation  industry  must  not  be  allowed  to  decay 
through  any  neglect  of  studies  in  this  direction.  It  must  not, 
however,  be  forgotten  that  whilst  our  British  plantations 
may  be  maintained  in  the  way  indicated,  the  problem  is 
not  only  one  of  industrial  botany,  but  depends  on  the 
suitability  of  the  product  for  the  markets  of  the  world,  and 
here  we  depart  from  the  botanical  cultivation  of  the  plant 
to  the  chemical  and  physical  properties  of  the  latex,  and  the 
methods  adopted  for  its  coagulation  and  ultimate  vulcaniza- 
tion by  the  home  user. 

To  correlate  these  several  factors  is  no  easy  task  ;  and  the 
Rubber  Growers'  Association  and  the  laboratories  here  and 
in  the  East  have  long  investigations  in  front  of  them,  and  it 
is  doubtful  if  the  planter,  relying  on  the  information  to  be 
obtained  from  botanical  experimental  cultivation,  will 
succeed  in  improving  his  position,  unless  the  subsequent 


THE  RUBBER  HYDROCARBONS 


fate  of  his  product  is  known.  There  is  at  the  present  time 
considerable  lack  of  knowledge  as  to  the  causes  of  the 
variability  of  plantation  Para  rubber  with  different  technical 
mixings,  so  that  until  we  know  what  are  the  conditions  of 
growth  which  cause  the  latex  to  have  a  different  rate  of 
cure  when  coagulated,  further  progress  in  this  direction 
must  be  slow. 

At  Kuala  I^umpur,  the  Department  of  Agriculture  of 
the  Federated  Malay  States  has  done  good  work  in  this 
direction  during  the  last  few  years  by  obtaining  evidence 
which  goes  to  show  that  the  latex  contains,  in  addition  to 
the  rubber,  proteid  substances  which  modify  the  rate  of 
cure,  and  that  these  substances  are  not  precipitated  by 
ordinary  coagulation. 

To  obtain  uniformity  in  first  latex  rubbers,  the  nature 
of  this  proteid,  its  amount,  and  in  fact  all  the  conditions  of 
its  formation  in  relation  to  the  latex  production,  must  be 
ascertained  before  further  progress  in  standardization  can 
be  effected. 

Buyers  know  that  an  over-smoked  rubber  has  lost  much 
of  its  value,  and  planters  seldom  now  err  in  this  direction. 
Besides  over-smoking,  over-washing  and  over-machining 
and  the  excessive  use  of  sulphite  and  preservatives  con- 
tribute to  the  destruction,  or  prevent  the  formation  of  the 
catalytic  proteids  which  seem  so  essential  for  rubber  to 
behave  well  on  vulcanization. 

As  a  matter  of  fact,  the  Kuala  lyUmpur  Agricultural 
Station  is  now  engaged  on  the  study  of  these  problems,  and 
useful  experimental  work  is  being  done  in  the  East,  and  at 
Delft,  in  Holland,  under  the  Dutch  Government ;  but  there 
is  need  for  comparison  of  the  work  and  correlation  of  the 
results  obtained  at  these  different  centres  of  investigation 
with  the  Ceylon  research  and  that  which  is  going  on  in 
Africa,  more  especially  at  Aburi. 

It  is  only  quite  recently  that  Whitby  has  again  made  a 
comparison  of  the  Brazilian  and  plantation  methods  of 
preparing  Para  rubber,  and  from  the  results  of  his  investiga- 
tions it  seems  clear  that  the  Brazilian  method  of  preparing 


4   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


rubber  by  smoke  coagulation  is  not  superior  to  the 
plantation  method  of  preparing  it  as  smoked  sheet.  He 
also  shows  that  the  latex  from  young  trees  is  not  inferior 
to  that  from  old  trees,  in  fact,  the  rubber  prepared  from 
the  latex  of  young  trees  proved  superior  to  that  from  the 
latex  of  old  trees,  both  when  prepared  as  smoked  sheet 
and  when  prepared  by  smoke  coagulation  in  the  Brazilian 
fashion. 

The  experimental  botanical  stations  of  our  plantations 
in  the  tropics  cannot  also  afford  to  ignore  the  work  of  the 
English  and  German  chemists  on  the  synthetic  production 
of  rubber.  Synthetic  rubber  exists,  and  was  probably  of 
use  to  a  limited  extent  during  the  war,  just  as  synthetic 
nitrates  exist  and  have  rendered  the  war  possible,  although 
we  still  look  to  the  tropics  for  both  these  materials. 

The  chemist  can  demonstrate  the  conversion  of  starch 
into  isoprene  or  butadiene,  and  Harries  is  confident  that  the 
rubber  obtained  from  isoprene  with  the  aid  of  acetic  acid  in 
the  laboratory  is  identical  with  that  coagulated  from  the  plant 
latex  by  the  same  acid  in  the  field.  The  botanist  has  yet 
to  tell  us  how  in  Brazil,  or  in  Malay,  the  plant  converts  its 
starch  into  the  isoprene  polymer. 

It  may  be  held  that  the  natural  rubber  is  elaborated 
from  cellulose  itself  and  that  starch  is  not  the  elemental 
source  of  the  rubber  latex.  Sawdust  under  pressure  can 
be  hydrolysed  to  Isevulinic  acid,  and  this  substance  has  in 
the  laboratory,  by  the  aid  of  phosphorus  trisulphide,  been 
first  converted  into  methyl  thiophene  which  loses  its  sulphur 
when  heated  with  copper  in  a  stream  of  hydrogen  and  forms 
isoprene.  Although  the  plant  brings  about  these  changes 
in  an  unknown  way,  it  certainly  accomplishes  the  task  in 
a  more  economic  manner ;  so  that,  at  the  present  time, 
the  natural  product  takes  the  field  and  gives  us  another 
example  of  the  advantages  which  nature  has  over  the 
products  of  the  laboratory  when  accurate  knowledge  of 
scientific  agriculture  is  brought  to  bear  upon  tropical  plant 
development. 


THE  RUBBER  HYDROCARBONS 


The:  Raw  Materiai^s  :  their  Formation  in  Nature, 
THEIR  Distribution 

It  is  difficult  to  give  a  scientific  definition  of  rubber. 
An  elementar>^  chemical  analysis  shows  that  it  has  the 
empirical  formula  C5H8  ;  that  is,  identical  with  that  of  the 
large  class  of  naturalty-occurring  substances  known  as 
terpenes.  This  does  not,  however,  represent  the  molecular 
composition,  and  as  far  as  the  naturally  occurring  rubber  is 
concerned  this  has  not  yet  been  definitely  determined.  The 
main  reason  for  this  is  that  rubber  is  a  colloid. 

By  destructive  distillation  of  coagulated  rubber,  simple 
hydrocarbons  of  definite  and  known  composition  have  been 
obtained,  and  it  is  probable  that  the  rubber  molecule  is  a 
condensed  or  polymerized  form  of  these  hydrocarbons. 
Our  knowledge  on  this  point,  however,  is  still  very  obscure 
and  therefore  uncertain  and  contradictory. 

The  subject  is  further  dealt  with  under  synthetic 
rubber. 

Origin  and  Distribution.— The  material  commonly 
known  as  rubber  does  not  occur  naturally  as  such,  but  is 
obtained  from  a  fluid  known  as  latex  which  is  secreted  by 
many  plants — the  so-called  rubber  plants.  The  fluid  flows 
out  of  the  plant  when  the  latter  is  wounded  or  tapped,  and 
by  the  addition  of  acids  and  heat  it  becomes  coagulated  and 
forms  what  we  recognize  as  rubber.  From  an  economical 
point  of  view  the  formation  of  the  latex  in  the  plant,  its 
distribution  and  its  function  is  of  the  greatest  importance 
and  will  therefore  be  briefly  dealt  with  here. 

The  latex  is  contained  in  the  cellular  tissue  just  outside 
the  ring  of  cambium  and  is  distributed  in  that  portion  of 
the  plant  by  the  so-called  laticiferous  vessels.  These  form 
a  complex  anastomosing  network,  better  seen  when  a 
longitudinal  section  is  cut.  The  cambium  layer,  which  is 
adjacent  to  the  laticiferous-bearing  tissue,  is  the  most  im- 
portant part  of  the  plant,  as  growth  within  and  without 
takes  place  from  this  layer.    If , '^therefore,  on  tapping  a 


6    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


rubber  tree  the  cambium  is  damaged  by  too  deep  a  cut, 
irreparable  damage  is  made  to  the  plant.  Any  form  of 
tapping  must  therefore  avoid  the  cambium,  and  preferably 
should  aim  at  getting  latex  from  only  a  portion  of  the 
laticiferous  vessel  in  order  that  the  function  which  the 
vessels  play  in  the  life  of  the  plant  may  be  interfered  with 
as  little  as  possible.  Up-to-date  tapping  methods  recognize 
this  and  various  instruments  are  in  the  market  which  more 
or  less  meet  with  these  requirements. 

lyike  the  fluids  in  all  plants,  the  latex  is  primarily  formed 
from  carbon  dioxide  and  moisture  by  the  influence  of  sun- 
light, heat,  enzymes  and  catalytic  agents,  and  various 
hypotheses  have  been  put  forward  to  explain  how  the  rubber 
hydrocarbons  are  synthetized  by  the  plant. 

Experiment  has  shown  that  the  green  colouring  matter 
present  in  the  leaves  of  plants  is  the  active  agent  in  the 
decomposition  of  carbon  dioxide  and  that  the  change  cannot 
be  brought  about  by  the  colourless  protoplasm.  The  pre- 
liminary stage  is  accompanied  by  the  liberation  of  oxygen, 
and  formation  of  formaldehyde — 

CO2  +H2O  =02  +HCHO 

The  reaction  does  not  proceed  so  simply,  and  hydrogen 
peroxide,  together  with  other  oxidases  like  percarbonic  acid, 
are  no  doubt  also  produced,  thus,  according  to  Bach — 

3H20+3C02=2(H2C04)  +HCHO 
2H2CO4  =-2C02  +2H2O2 

-2CO2+2H2O+O2 

All  the  products  have  been  identified  in  various  plants. 

The  work  of  Butlerow  and  others  has,  however,  shown 
that  formaldehyde  is  converted  into  carbohydrates — 
notably  fructose  ;  and  the  presence  of  sugary  substances 
in  most  rubber  serums  at  once  suggests  the  possibility  of 
rubber  being  derived  from  a  sugar. 

Now,  erythritol,  a  direct  sugar  derivative,  is  easily  con- 
verted by  the  action  of  formic  acid  into  ery threne  or  butadiene 
from  the  methyl  derivative  of  which  synthetic  rubbers, 


THE  RUBBER  HYDROCARBONS 


very  similar  in  character  to  natural  rubbers,  have  been 
obtained  in  the  laboratory — 

CH2OH  -CHOH  -CHOH  -CHsOH^CHs =CH  -CH  -CHs 

Further,  according  to  Harries,  when  rubber  is  oxidized 
by  means  of  ox>^gen  or  ozone,  laevulinic  aldehyde  is  formed, 
and  this  is  identical  with  the  product  formed  when  carbo- 
hydrates such  as  starch  are  treated  with  acids.  This 
aldehyde  contains  the  C5H3  grouping. 

Function  of  Latex  in  the  Plant. — The  exact  function 
which  latex  plays  in  the  life  of  the  plant  is  not  known,  and 
several  views,  all  more  or  less  contradictory,  have  been  put 
forward  to  explain  the  work  which  the  latex  is  called  upon  to 
perform.  The  solution  of  the  problem  is  very  important 
from  the  practical  standpoint,  for  if  the  latex  is  considered 
to  be  a  waste  product  of  the  plant  its  entire  removal  would 
be  without  injury  to  the  plant.  On  the  other  hand,  if  the 
latex  has  an  important  functional  purpose  to  fulfil,  then 
tapping  operations  in  the  plant  must  be  careftdly  performed 
in  order  not  to  drain  the  plant  and  thus  imperil  its 
existence. 

The  most  recent  and  extensive  investigations  have  been 
carried  out  by  Vernet,  who  has  studied  the  subject  very 
thoroughly,  and  he  arrives  at  certain  definite  conclusions. 
His  views  and  experiments  are  briefly  put  out  as 
follows  : — 

Is  the  latex  an  excretory  product  ?  Plant  secretions 
are  usually  products  which  the  plant  diverts  into  large 
cavities,  where  they  remain  imprisoned  during  the  whole 
life  of  the  plant  out  of  all  contact  with  other  liquid  food 
materials. 

Latex  vessels,  however,  are  far  from  being  closed  vessels. 
They  form  a  continuous  network  in  which  the  meshes  are  all 
inter-connected.  Further,  the  quantity  of  latex  varies  very 
considerably  at  different  times,  and  this  is  a  most  unusual 
occurrence  if  the  latex  be  regarded  as  an  excretionary 
product.'' 

If  the  latex  were  an  excretion,  then  it  could  be  removed 


8    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


without  causing  injury  to  the  plant.  To  answer  this 
question  Vernet  completely  severed  the  latex  vessels  from 
a  Hevea  tree  and  found  that  it  entirely  lost  its  leaves  and 
therefore  suffered  from  the  treatment.  The  latex  cannot 
therefore  be  an  excretion  whose  presence  is  immaterial  to 
the  plant. 

Having  thus  shown  that  latex  is  not  an  excretion  Vernet 
definitely  asserts  that  the  latex  as  a  whole  is  a  food 
indispensable  to  the  life  of  the  tree,  and  confirms  his 
evidence  by  analogy,  chemical  composition  and  effects  of 
tapping. 

Analogy. — The  laticiferous  vessels  are  morphologically 
very  similar  to  sieve  tubes  which  are  the  recognized  organs  for 
nutrition  and  circulation  in  all  plants.  The  vessels  are  more 
numerous  near  the  generative  or  cambium  layer,  just  where 
one  would  expect  the  nutritive  function  to  be  most  necessary, 
and  are  found  in  all  stages  of  the  plant's  existence,  even 
in  the  embryo  of  the  seed  and  the  j^oung  germinating 
plant. 

Chemical  Composition. — The  latex  besides  containing 
water  and  rubber  contains  also  sugars  and  proteins,  i.e. 
materials  indispensable  to  the  nutrition  of  plants. 

Tapping. — The  rate  of  increase  in  girth  of  a  tree  which 
follows  upon  tapping  is  less  than  normal,  but  becomes  more 
than  normal  when  tapping  ceases.  The  seeds  also  lose  in 
weight  after  tapping.  All  the  above  facts  go  to  show  that 
the  formation  of  latex  is  a  continuous  function  of  a  plant's 
life-history.  The  flow  is  not  continuous,  and  after  tapping 
naturally  slackens  down,  thus  promoting  coagulation  of 
what  is  left  on  the  wounds  and  so  stopping  the  process  of 
bleeding.  The  latex  thus  serves  a  protective  function  in 
addition  to  that  of  nutrition. 

Rubber-bearing  Species. — The  number  of  plants 
which  yield  a  rubber  containing  latex  is  very  great  and 
belongs  to  four  natural  orders — Euphorbiaceae,  Artocarpeae, 
Apocynaceae  and  Asclepiadeae.  The  plants  vary  con- 
siderably in  size  ;  some  are  herbaceous  and  contain  latex  in 
their  roots  or  stems,  while  others  are  climbers  or  vines  and 


THE  RUBBER  HYDROCARBONS  9 


yield  latex  from  the  branches.  Others,  again,  are  shrubs ; 
but  the  bulk  from  which  the  rubber  of  commerce  is  obtained 
are  forest  trees  of  considerable  size. 

The  tree  which  gives  the  best  rubber  is  Hevea  Braziliensis, 
which  grows  wild  in  the  basin  of  the  Amazon,  Brazil.  The 
tree  is  the  most  important  of  the  order  Euphorbiacese, 
grows  to  a  height  of  50  to  65  feet,  and  the  trunk  attains  a 
diameter  of  60  to  70  inches.  Rubber  was  first  discovered 
from  this  tree,  and  the  method  of  tapping  and  coagulating 
the  latex  has  remained  unchanged  even  up  to  the  present 
day  in  its  native  habitat.  The  tapping  consists  in  forming 
an  incision  on  the  tree  about  6  feet  from  the  ground  and 
collecting  into  cups  the  latex  which  immediately  oozes  out. 
This  is  then  poured  in  a  thin  stream  on  the  flat  part  of  a 
paddle,  which  is  rotated  on  a  smoky  fire.  In  this  way  the 
latex  coagulates  at  once,  and  forms  a  layer  on  the  paddle, 
and  when  by  continual  rolling  and  pouring  of  latex  the 
layers  reach  a  weight  of,  say,  20  lbs.  the  rubber  is  branded 
and  sold  as  Fine  Para.'*  This  still  obtains  the  best  price. 
The  number  of  Hevea  trees  in  the  Amazon  valley  is  estimated 
at  200  million,  of  which  a  half  are  being  tapped.  Other 
species  from  which  rubber  is  obtained  are — Castilloa  and 
Manihot  trees  and  Guayule  shrubs,  all  in  South  and  Central 
America,  where  the  method  of  collection  and  coagulation  is 
practically  the  same  in  all  these  cases. 

In  Africa  the  rubber-yielding  plants  belong  mainly  to 
the  order  Apocynaceae,  and  include  trees  like  Funtumia  as 
well  as  shrubs,  creepers  and  plants  with  rhizomes.  Each 
of  these  species  of  plant  requires  its  own  method  of  treatment 
for  the  winning  of  the  rubber  latex,  but  all  the  methods 
are  unfortunately  crude  and  have  been  adopted  with  no 
eye  for  the  future  either  of  the  plant  or  the  Rubber 
industry. 

The  rubber  from  the  herbaceous  plants  is  obtained  by 
drying  the  roots  in  the  sun,  cutting  them  into  small  pieces 
and  chipping  off  the  bark  by  striking  with  a  mallet.  The 
pieces  of  bark  are  next  treated  in  the  same  manner  and  the 
latex  which  issues  from  the  tissues  allowed  to  dry  into 


10    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


masses.  These  are  cut  up,  purified  by  putting  into  several 
changes  of  boiUng  water,  and  then  massed  into  sheets.  In 
the  case  of  creepers,  the  stems  are  cut  into  pieces  and  the 
latex  which  exudes  from  the  cut  ends  collected  in  a  hole 
dug  in  the  ground. 

In  Asia  the  natural  rubber  plants  differ  entirely  from 
those  in  America  and  Africa  in  that  they  yield  very  resinous 
rubbers.  In  fact,  very  few  rubber  trees  may  be  said  to  exist 
here,  although  climate  and  soil  appear  quite  appropriate  to 
the  Hevea  type  of  plant.  This  being  so,  attempts  were  made 
some  thirty  or  forty  years  ago  to  cultivate  the  ordinary 
rubber  plants  in  this  portion  of  the  globe.  Hevea  Braziliensis 
being  the  plant  which  yields  the  best  rubber,  was  chosen  as 
the  best  type  of  plant  to  introduce,  and  extensive  plantations 
were  laid  down.  Many  difiiculties  were  encountered,  one 
very  important  one  being  that  the  seeds  quickly  lost  their 
germinating  power  and  had  therefore  to  be  planted  and 
allowed  to  germinate  during  the  voyage.  The  trees  also  do 
not  give  a  profitable  yield  of  rubber  till  they  are  more  than 
five  years  old,  so  that  the  risk  attending  failure  was  great. 
The  yields,  hov/ever,  increase  as  the  plants  grow  older  and 
may  reach  sj  to  4J  lbs.  per  annum.  Attempts  have  also 
been  made  to  cultivate  Manihot  Glazovii  as  this  lends  itself 
to  an  easy  method  of  cultivation,  but  although  the  rubber 
obtained  from  it  is  good,  the  yield  is  poor,  not  exceeding 
18  ozs.  per  annum.  In  spite  of  preliminary  setbacks  the 
plantations  have  been  a  great  success  and  many  millions  of 
capital  have  been  sunk  in  them.  The  number  of  Hevea 
plantation  trees  was  estimated  at  60  millions  some  years  ago, 
and  the  weight  of  rubber  produced  in  1911,  14,000  tons  as 
compared  with  38,000  tons  from  the  Amazon  ;  but  in  igig 
some  320,000  tons  of  rubber  was  the  actual  world's  consump- 
tion, showing  the  very  rapid  development  in  the  plantation 
industry  within  the  last  decade.  The  United  States  shows 
an  average  annual  increase  of  277  per  cent,  during  this 
period.  The  output  from  Brazil  has  fluctuated  from 
42,000  to  26,750  tons  per  annum,  and  wild  rubber  has  not 
exceeded  30,000  tons  per  annum, 


THE  RUBBER  HYDROCARBONS  ii 


The  following  table  shows  the  rate  of  increase  in  the 
world's  rubber  production  during  the  last  three  years 


From  plantations 
(long  tons). 

Other  sources 
(long  tons). 

Total 
(long  tons). 

Average  price  per 
lb.  in  London. 

I9I6 

152,650 

48,948 

201,598 

2S.  g^d. 

I9I7 

213,070 

52,628 

265,698 

25.  g^d. 

I9I8 

200,950 

40,629 

241,579 

25.  3J^^. 

and  this  shows  at  once  the  growing  importance  of  plantation 
rubber  in  the  world's  market. 

The  plantation  in  Malaya  alone  produced  177,000  tons 
in  1919,  while  the  Netherlands  Indies  produced  some  76,000 
tons.  The  area  under  plantation  is  estimated  at  2,760,000 
acres  of  which  only  2,000,000  are  of  bearing  age.  In  1920 
the  consumption  is  estimated  at  350,000  tons,  with  an  over- 
production of  34,000  tons,  and  with  a  steady  increase  in 
plantation  output  it  is  anticipated  that  the  figure  for  1924 
will  be  640,000  tons. 

By  far  the  greatest  consumer  of  the  world's  rubber  is  the 
United  States,  and  the  following  table  shows  that  nearly 
three-fourths  of  the  world's  production  is  manufactured  in 
that  country.  Great  Britain,  which  virtually  commands 
all  the  plantation  areas,  comes  second,  using  only  one-tenth 
of  the  world's  supply. 


Rubber 

Productive 

Consumer. 

Population. 

consumed 

Per  cent. 

value. 

(tons). 

£ 

United  States 

92,000,000 

177,088 

69-0 

177,000,000 

Great  Britain 

45,000,000 

25,983 

IO-2 

26,000,000 

France 

40,000,000 

17,000 

67 

17,000,000 

Italy  

35,000,000 

9,000 

3*5 

9,000,000 

Russia 

174,000,000 

7,500 

3'o 

7,000,000 

Canada 

7,000,000 

6,287 

27 

6,300,000 

Scandinavia   . . 

5,323 

1-9 

4,500,000 

Japan  and  Australia  . . 

4,500 

1-8 

3,000,000 

Germany  and  Austria 

117,000,000 

H,ooo 

1*2 

12    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Tapping 

The  methods  for  tapping  the  rubber  have  undergone 
considerable  changes  during  the  ten  years  in  which  systematic 
production  in  the  East  has  been  conducted.  Daily  tapping 
was  felt  to  be  too  great  a  strain  upon  the  tree,  and  thus 
many  estates  adopted  tapping  on  alternate  days,  and  on 
some  a  three-day  period  was  adopted.  The  number  of  cuts 
per  day  were  also  altered  until  most  estates,  attracted  by 
the  good  yields,  considered  that  two  superimposed  cuts  on 
one-quarter  of  the  tree  were  not  too  much,  but  lately,  owing 
to  the  war  and  a  desire  to  restrict  production,  one  cut  per 
day  on  one-quarter  has  come  more  into  vogue.  There  is  no 
doubt  that  a  conservative  method  of  daily  tapping  will 
prolong  the  available  life  of  the  tree  and  give  ample  time  for 
bark  renewal.  The  cuts  should  be  sufficiently  close  to  take 
up  not  more  than  an  inch  of  bark  per  22  cuts. 

PhysicaIv  and  Chkmicai,  Properties  of  IvATEx 

Physical  Properties. — The  latex  of  a  rubber  tree  is  a 
white  or  pale  yellow  milky  liquid,  varying  in  consistency 
according  to  the  rubber  it  contains.  It  stains  the  hands 
black,  develops  an  odour  of  methylamine  on  standing  and 
has  an  alkaline  reaction.  Its  specific  gravity  ranges  from 
•905  to  1*041,  the  average  being  about  i*oi8,  corresponding 
to  a  rubber  content  of  about  32  per  cent.  Speaking 
generally,  the  lower  the  specific  gravity  the  greater  the  per- 
centage of  rubber,  and  tables  have  been  drawn  up  from 
which  the  rubber  content  is  obtained  directly  by  referring 
to  the  gravity  at  a  definite  temperature.  The  metrolax  " 
gives  the  percentage  of  rubber  in  the  latex  as  shown  by  its 
gravity. 

The  rubber  in  the  latex  exists  in  the  form  of  nearly 
spherical  globules,  having  an  average  diameter  of  one- 
thousandth  of  a  millimetre,  and  when  observed  vuider  the 
high  power  of  a  microscope  exhibit  the  well-known  Brownian 


THE  RUBBER  HYDROCARBONS 


mov€iment.  The  movement  is  very  irregular,  and  cinemato- 
graph films  show  that  the  path  traced  out  by  each  particle 
is  a  long  one.  The  eftect  of  the  addition  of  chemicals  has 
been  studied.  Soda  has  but  little  effect  on  it.  Acetic  acid 
completely  holds  up  the  movement  even  before  coagulation 
sets  in.  A  20  per  cent,  solution  of  sodium  chloride  arrests 
the  movement  completely,  and  the  number  of  particles 
counted  under  these  conditions  amounts  to  50  million  per 
cubic  millimetre.  Salt  has  been  used  as  a  coagulant  in  East 
Africa. 

Direct  experiment  has  shown  that  the  rubber  content  of 
latex  varies  with  the  height  from  which  the  tree  is  tapped, 
and  also  with  the  age  of  the  tree.  Thus  from  a  Ficus  plant 
7  ft.  4 1  ins.  high  Adriani  found — 

At  12  ins.    latex  contained  25  per  cent,  rubber 
„  5  ft.  6  ins.   „       „  24 
„  6  ft.  10  ins.  „       „  20 
„  the  top       „       „  17 

At  6  years  the  latex  of  a  Castilloa  tree  contains  20  %  rubber 

>>  1     >»       9f       »j       }}       >>        /o  3) 

i>  ^  iy  yy  a  >f  >f  ^9   /o  >> 

if   9  >>  y>  >>  3-^    /o  ft 

Chemical  Properties. — An  average  chemical  composition 
of  the  latex  of  a  full-grown  Hevea  tree  shows  that  it  contains — 


Water 

.  •  55*0 

Rubber  . . 

41*0 

Sugars  . . 

•8 

Albuminoids 

2-8 

Mineral  Matter 

•4 

100*0 

In  addition,  small  quantities  of  oxidases,  resins  and 
sulphates  are  present.  On  standing,  coagulation  takes 
place,  and  the  rubber  separates  as  a  colloid  and  leaves  a 
clear  serum.  Hevea  latex  is  alkaline  in  reaction  as  it  issues 
from  the  tree,  but  becomes  acid  after  a  little  time,  owing 


14   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


probably  to  fermentation  of  the  sugars  present.  The  latex 
becomes  acid  in  very  much  the  same  way  as  milk  sours,  and 
coagulation  then  takes  place  under  the  influence  of  the  acid 
formed.    Most  other  latices  are,  however,  acid  as  tapped. 

Latex  can  be  kept  without  alteration  for  about  twelve 
hours.  Beyond  this,  watering  or  the  addition  of  chemicals 
is  necessar}^  to  prevent  chemical  change  taking  place. 

Concentrated  acetic  acid,  formic  acid,  citric  acid,  and 
other  strong  organic  acids  bring  about  coagulation  very 
rapidly.  The  globules  join  up,  forming  chains  similar  to 
streptococci ;  they  swell  to  twice  their  volume  and  then 
separate  out  together  as  a  coagulum. 

Acetic  acid  is  the  usual  coagulant  employed  in  plantation 
areas,  but  citric  and  formic  acids  are  also  used.  Hydror 
chloric  acid  is  a  very  powerful  coagulant,  a  current  of  gas 
producing  coagulation  almost  at  once,  and  rubbers  which 
are  inclined  to  be  tacky  lose  this  tendency  by  this  treatment. 
If  the  treatment  is  prolonged  chemical  action  proceeds 
further.  The  rubber  hardens  and  turns  to  a  mass  resembling 
ebonite.  Carbon  dioxide  is  also  an  excellent  coagulant  and 
no  doubt  is  the  active  agent  in  the  out-of-date  smoke 
coagulation  still  carried  out  in  the  Amazon. 

The  threatened  shortage  of  acetic  acid  in  the  plantations 
has  caused  attention  to  be  given  to  natural  or  spontaneous 
coagulation,  and  the  method  introduced  by  Maude  and  Cross 
— the  so-called  M.C.T.  process — in  which  separation  of 
rubber  from  latex  is  allowed  to  take  place  under  the  anaerobic 
conditions  produced  by  the  carbon  dioxide,  has  been  com- 
mented on  very  favourably,  and  is  said  to  yield  a  rubber 
which  in  its  degree  of  uniformity  is  superior  to  any  other 
form  of  plantation  rubber. 

Sodium  chloride,  mercuric  chloride  and  phenol  precipitate 
rubber  from  its  latex  and  all  protein  precipitants  behave 
towards  latex  as  coagulants.  Formaldehyde,  very  curiously, 
has  little  action  even  when  heated.  The  author  suggested 
acid  sulphate  of  soda  as  a  coagulant  during  the  war. 

The  carbohydrates  and  glucosides  which  occur  in  the 
serum  of  rubber  latex  have  received  a  great  deal  of  attention; 


THE  RUBBER  HYDROCARBONS  15 


and  the  presence  of  the  C5H8  group,  or  multiples  of  it,  which 
has  been  definitely  established  in  them,  points  to  the  fact  that 
rubber  is  very  probably  elaborated  by  the  plant  from  these 
sugars. 

The  work  of  Aime  Girard  in  this  connection  is  worthy  of 
mention.  Working  with  the  serum  of  fresh  latices  he 
obtained  coloured  masses  after  evaporation,  and  these  on 
extraction  with  alcohol  produced  white  colourless  cr37^stals 
which  he  called  Dambonite.  This  compound  has  the 
empirical  formula  C4H8O8,  melts  at  igo""  C.  and  volatilizes 
at  about  210°,  forming  long  needles.  It  does  not  reduce 
Fehling's  solution  nor  undergo  alcoholic  or  lactic  fermenta- 
tion. It  is  attacked  by  strong  acids  (HI)  with  the  formation 
of  methyl  iodide  and  a  neutral  substance  having  a  sweet 
taste,  a  fine  crystalline  structure  and  the  composition  of  a 
dehydrated  glucose.    This  substance  he  called  Dambose. 

Similar  sugars  were  obtained  from  the  latices  of  other 
rubber  plants,  and  lastly,  in  1911,  Pickles  and  Whitfield 
succeeded  in  proving  the  presence  of  dambonite  in  Hevea 
latex. 

The  presence  of  sugars  in  the  latices  of  all  rubbers  thus 
strengthens  the  belief  that  rubber  is  a  sugar  derivative. 

The  mineral  salts  in  rubber  latex  are  very  small  in 
amount  and  have  the  normal  composition,  K,  Ca,  Mg,  and  Na 
salts  having  been  identified.  Oxalates  have  also  been  found. 
Acidity  is  usually  very  small.  The  resins  present  are 
separated  by  extraction  with  alcohol  or  acetone,  but  very 
little  is  known  about  them.  They  are  optically  active  and 
are  precipitated  with  rubber  on  coagulation.  In  some  plants 
the  percentage  of  resin  is  greater  than  that  of  rubber,  and 
their  extraction  from  the  latex  before  instead  of  after  coagula- 
tion would  produce  a  better  quality  rubber. 

All  latices  examined  have  shown  the  presence  of  proteins, 
and  these  differ  from  ordinary  proteins  in  being  very  slowly 
and  incompletely  precipitated  when  heated.  The  proteins 
behave  normally  with  all  alkaloidal  precipitants  and  protein 
coagulants,  and  experiment  has  shown  that  coagulation  of 
latex  is  really  a  coagulation  of  proteins  and  not  of  rubber 


i6   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


itself.  This  question,  in  fact,  involves  that  of  the  presence 
of  rubber  as  such  in  the  latex,  and  Preyer,  Weber,  and  others 
have  shown  that  rubber  separated  from  latex  by  a  centrifuge 
is  not  precipitated  by  coagulants.  Further,  the  addition 
of  ether  to  latex  does  not  extract  a  solid  hydrocarbon,  but 
on  evaporation  of  the  ether  a  thick  oil  is  first  obtained  which 
only  after  some  time  solidifies  to  pure  rubber. 

The  coagulated  latex  after  washing  is  marketed  in  the 
form  of  crepe  or  smoked  sheet.  Bisulphite  of  soda  is  usualty 
added  to  produce  a  pale  coloured  crepe.  Various  devices 
are  used  for  recovering  the  scrap  rubber  adhering  to  the 
trees,  which  appears  on  the  market  as  bark  scrap  and 
compo.'' 

The  drying  and  smoking  of  the  sheets  have  to  be  carefully 
attended  to  so  as  to  ensure  uniformity  and  freedom  from 
spots  or  discoloration,  which  detract  from  the  value  of  the 
finished  sheet. 

ChkmicaIv  and  Physicai.  Properties  and  Testing  of 

Rubber 

Chemical  Properties  and  Composition. — Before  ex- 
amining the  chemical  properties,  the  purity  of  the  rubber 
must  be  established.  Resins,  protein,  etc.,  must  be  removed, 
and  when  this  is  done  the  question  still  remains  as  to  whether 
the  product  obtained  is  a  simple  substance.  Unfortunately, 
rubber  has  no  definite  melting  point,  neither  have  any  of 
its  derivatives  with  the  possible  exception  of  the  ozonides 
obtained  by  Harries. 

Neglecting  these  preliminary  handicaps  and  proceeding 
as  with  other  organic  substances  one  finds  that  carbon  and 
hydrogen  are  alone  present,  and  that  the  empirical  formula 
corresponds  with  CsHg.  The  molecular  weight  has  not  been 
determined  with  certainty,  as,  like  all  colloid  substances,  it 
has  no  osmotic  pressure  in  solution,  and  freezing-point  and 
boiling-point  methods  are  useless.  The  rubber  molecule  is 
no  doubt  a  large  one,  and  its  formation  is  due  to  the  union 
of  nuclei  of  5  to  10  carbon  atoms.    According  to  Bary  and 


THE  RUBBER  HYDROCARBONS 


Weidert  vulcanization  is  the  addition  of  one  atom  of  sulpliur 
at  each  end  of  a  chain  of  CioHig  nuclei ;  thus  (CioHi6)nS2  ; 
and  as  2*5  per  cent,  is  the  amount  of  sulphur  required  to 
bring  this  about,  it  follows  that  the  figure  for  (CioHie)^ 
must  correspond  to  about  2500.  This,  however,  does  not 
agree  with  the  molecular  weight  of  two  derivatives — the 
ozonide  and  the  nitrosite,  which  contain  10  and  20  carbon 
atoms  respectively,  and  as  there  is  no  reason  why  the 
parent  substance  should  contain  any  more,  it  is  very 
probable  that  this  also  contains  the  same  number  of  carbon 
atoms. 

The  rubber  molecule  is  thus  an  unsaturated  hydrocarbon, 
but  the  most  satisfactory  proof  of  the  presence  of  unsaturated 
linkages  is  obtained  from  the  work  of  Harries,  who  applied 
his  method  of  ozonization  previously  used  to  settle  the 
formulae  of  oleic  acid  and  other  unsaturated  compounds. 
When  ozone  is  passed  into  a  chloroform  solution  of  pure 
rubber  an  explosive  oil  is  obtained  which  solidifies  in  vacuo 
and  melts  at  50°  C,  and  the  molecular  weight  of  this  com- 
pound corresponds  to  CioHigOe.  But  previous  investiga- 
tions have  shown  that  each  three  atoms  of  oxygen  are 
attached  to  one  double  bond,  hence  there  can  only  be  two 
such  linkages  for  every  ten  carbon  atoms.  More  recent 
work  has  shown  that  rubber  in  a  similar  manner  gives  a 
diozonide. 

Having  established  the  presence  of  two  double  bonds, 
Harries  set  about  to  determine  their  position  in  the  molecule. 
This  he  did  by  hydrolysing  the  ozonide  with  steam  and 
examining  the  product  obtained.  By  this  means  he  was 
able  to  show  that  the  ozonide  on  hydrolysis  produced 
hydrogen  peroxide  and  laevulinic  aldehyde,  and  the  latter 
then  oxidizes  to  laevulinic  acid.    Thus — 

Aldehyde. 

CioHieOe  +H2O  =2  (CH3.CO.CH2.CH2.CHO)  +H2O2 
CH3.CO.  (CH2)  2.CHO  +H2O2     CH3.CO.  (CH2)  2.COOH+H2O 

Acid. 


The  changes  take  place  at  the  double  linkage,  thus — 
s.  2 


l8    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 

(i)    RiCH=CHR2+03     RjCH  CHRg 

I  I 


RiCH  CHR2 

I  1 
0—0—0 

H— O— H 


0—0—0 
R1CHO+R2CHO+H2O2 


Under  certain  circumstances  peroxides  may  be  formed, 
thus — 

RiCH  CHR2         RiHC^  I  +OCHR2 

0—0—0 

From  the  ozonide  we  may  therefore  obtain — 

Laevulinic  Aldehyde, 
lyaevnlinic  Peroxide. 
I^aevulinic  Acid. 

To  explain  these  reactions  one  must  regard  the  ozonide 
molecule  as  a  cyclic  one,  for  if  it  were  a  chain  molecule,  the 
diozonide  would  give  two  hydrolysis  products  oxygenated 
at  only  one  end  of  their  chains  as  shown  above,  (i). 

The  formula  which  explains  all  these  reactions  is — 


CH3— 0 

/ 

/ 
O 


-CHo — CHo — CH 


o 


o 


o 


-0 


CH — CH  2 —  CH  2 — C — CH  3 
By  the  influence  of  steam  this  splits  up  as  shown  into 

CHs.CO.CHa.CHa.CHO  and    |  )>CH-CH2-CH2-C-CH3 

O^  /\ 
0—0 

Laevulinic  aldehyde.  Laevulinic  peroxide. 


THE  RUBBER  HYDROCARBONS  ig 


It  follows,  therefore,  that  the  original  hydrocarbon  must  have 
the  formula — 

CH3 — C  CH2 — CII2 — 

II  II 

CH — CH2 — CH2 — C — CH3 
i.c,  1*5  dimethyl  cyclo-octadiene  1-5. 

This  formula  further  explains  the  formation  of  isoprene  and 
dipentene,  which  are  obtained  on  distillation  thus — 

CH3-  C  CH2— CH2— CH  CH3— C— CH2— CH2— CH 

II  II  II  II 

CH — CH2 — CH2 — C.CH3      CH2  C — CH3 

I 

^  CH 

II 

CH2 
^0- 


CH3\  /CH2 — CH  <s  CH3\ 

>C— CH<  ^C.CHs   <-  >C.CH=CH2 


CH2  CH2  CH2  CH2 

Dipentene.  Isoprene. 

But  isoprene  can  be  polymerized  to  a  product  very  similar 
to  rubber  thus — 

CH3— C—  CH2  CH2=CH 

I  I 
CH=CH2        >^       CH2=C — CH3 

{CH3— C  CH2— CH2— CH 

II  II 

CH — CH2 — CH2 — C — CH3)ii 

Hence  the  definite  decomposition  products  and  deriva- 
tives can  be  explained  by  the  formula,  and  this  must  therefore 
be  taken  as  the  correct  one  for  the  time  being. 

Testing  of  Rubber. — A  chemical  analysis  of  rubber  is 
seldom  necessary,  but  is  required  when  mechanical  tests 
do  not  give  normal  figures. 

The  moisture  is  determined  by  drying  at  a  temperature 
of  90°  to  loo"^  F.  Resins  are  extracted  with  acetone,  the 
acetone  evaporated  and  the  resins  dried  and  weighed. 
Proteins  are  determined  by  multiplying  the  nitrogen  content 


20    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


as  obtained  by  the  ordinary  Kjeldahl  process  by  6*25. 
Matters  insoluble  in  benzene  or  some  other  solvent  are 
determined  and  considered  separately,  even  though  they 
contain  proteins.  Mineral  matter  is  determined  by  decom- 
posing and  burning  by  means  of  heat.  The  sum  of  these 
impurities  as  percentage  subtracted  from  100  gives  the 
rubber  as  a  difference  figure. 

Some  chemists,  however,  prefer  to  determine  the  rubber 
or  caoutchouc  by  estimating  the  amount  that  passes  into 
solution.  The  tests  on  which  one  bases  the  price  and  there- 
fore the  quality  of  the  rubber  are,  however,  all  mechanical 
tests,  and  in  recent  years  a  great  number,  based  on  new 
principles,  have  been  suggested  and  in  some  cases  adopted. 

Unfortunately,  the  method  most  commonly  practised 
consists  in  nothing  more  than  smelling,  seeing  if  it  tears 
when  stretched  between  two  hands  or  gives  way  to  a  strong 
push  of  the  finger  and  thumb.  Thus  more  definite  and 
reliable  methods  have  for  a  long  time  been  wanted. 

For  plantation  areas  the  following  tests  have  been 
suggested  : — 

1.  Viscosity  of  rubber  solutions  of  definite  strength. 

2.  Adhesion  tests,  A  solution  of  rubber  is  brushed  on 
a  piece  of  cloth  or  strong  paper  and  allowed  to  dry.  The 
dry  sheet  is  next  folded  and  the  two  surfaces  pressed  together 
and  made  to  adhere.  The  weight  required  to  tear  the  adher- 
ing surfaces  is  then  determined. 

3.  Tensile  strength.  Rubber  is  pressed  into  a  definite 
shape  during  cooling  or  drying  and  the  elongation  produced 
by  loading  weights  on  one  end  obtained. 

From  the  manufacturers'  point  of  view,  the  tests  must 
be  somewhat  modified  in  order  to  meet  the  various  uses  to 
which  the  rubber  is  to  be  put. 

The  most  favourite  is  the  tensile  test  which  determines 
the  breaking  stress  per  unit  of  cross-sectional  area  and  the 
elongation  at  rupture.  The  elongation  under  constant  load 
and  the  effect  of  varying  the  load  below  the  limit  of  breaking 
stress  are  also  determined.  All  these  tests  lay  bare  the 
mechanical  strength  of  rubber. 


THE  RUBBER  HYDROCARBONS 


21 


To  get  an  idea  of  the  resiliency  of  rubber  a  different 
series  of  tests  has  to  be  performed,  and  these  tests  determine 
the  Permanent  set  or  coefficient  of  resiliency  and 
Sub-permanent  set/'  The  permanent  set  is  the  permanent 
increase  in  length  after  the  full  retraction  which  follows  the 
withdrawal  of  a  stress.  If  this  is  measured  at  definite  intervals 
before  the  rubber  has  had  time  to  fully  retreat  we  then  get 
the  sub-permanent  set.  When  the  elongation  obtained  b}^ 
gradually  increasing  loads  and  the  retraction  by  gradually 
decreasing  the  loads  are  plotted,  an  hysteresis  curve  is 
obtained  and  the  form  of  a  series  of  such  curves  is  probably 
the  best  indication  of  the  quality  of  the  rubber. 

Manufacture  of  Rubber  Articles 

Before  the  rubber  can  be  used  for  manufacturing  pur- 
poses it  must  undergo  such  treatment  as  will  remove  any 
impurities  present  and  bring  it  to  a  form  suitable  for  the 
particular  article  required  to  be  made  from  it. 

The  removal  of  impurities,  which  usually  consist  of  more 
or  less  foreign  matter,  such  as  sand,  dirt,  bark  and  moisture, 
is  carried  out  by  the  process  known  as  washing.''  For 
this  purpose  the  raw  rubber  is  cut  into  lumps  or  slabs, 
steeped  in  warm  water  to  render  it  pliable  and  remove  some 
of  the  soluble  impurities,  and  then  passed  through  corrugated 
rollers.  A  stream  of  water  is  kept  running  on  the  rubber 
throughout  the  operation.  By  the  continued  squeezing  and 
disintegrating  action  of  the  rollers,  the  original  lumps  or 
slabs  of  rubber  are  converted  into  thin  corrugated  sheets, 
and  the  process  is  stopped  when  the  rubber  is  considered 
thin  and  clean  enough.  The  sheets  are  next  dried  on  racks 
in  a  drying  chamber  until  the  moisture  content  is  o '25-0*5  per 
cent.  The  drying  chambers  are  steam-heated,  but  the  cooler 
and  drier  the  atmosphere  is,  the  better  is  the  product 
obtained ;  otherwise  the  rubber  is  inclined  to  become 
tacky."  The  normal  drying  temperature  is  80 F.  and 
the  drying  takes  three  to  four  days,  if  ventilation  conditions 
are  suitable. 


22    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


When  dry  and  clean  the  rubber  is  made  to  undergo  the 
process  of  Mastication,  by  which  means  it  is  converted  into 
a  soft  doughy  mass.  The  process  is  carried  out  by  repeatedly 
passing  the  rubber  between  smooth,  hollow;  steam-heated 
rollers.  After  about  half  an  hour's  treatment,  the  rubber 
is  usually  of  the  required  consistency  and  ready  for  the  next 
process — Mixing,  Here  the  rubber  is  incorporated  with 
the  various  other  constituents  which  are  considered  necessary 
for  the  production  of  the  final  article.  Very  few  articles  are 
made  with  rubber  alone,  partly  on  account  of  its  price,  and 
partly  owing  to  its  excessive  softness  and  elasticity,  so  that 
the  mixing  process  is  one  which  must  be  carried  out  with 
practically  every  kind  of  rubber  article  manufactured.  The 
materials  other  than  rubber  and  sulphur  which  are  employed 
in  commercial  mixings  may  be  classified  as  follows  : — 

{a)  Cheapeners/'  These  include  such  substances  as 
powdered  chalk,  barytes,  ground  rubber  waste,  zinc  oxide, 
etc.,  and  are  added  for  no  other  purpose  than  to  cheapen 
the  final  article. 

(b)  Materials  added  for  a  definite  purpose,  such  as  (i)  to 
increase  mechanical  strength,  i.e.  toughen  or  harden  the 
goods — e.g.  zinc  oxide,  magnesia  (oxide  and  carbonate), 
lime  and  litharge.  (2)  To  improve  vulcanizing  conditions, 
e.g.  litharge,  magnesia,  quicklime,  antimony  pentasulphide 
or  any  other  polysulphide.  (3)  Colouring  matter,  e.g.  zinc 
oxide,  zinc  sulphide,  antimony  sulphide  (golden  and  crimson), 
mercury  sulphide  (vermilion),  cadmium  yellow,  chrome 
yellow,  chrome  green,  Prussian  blue,  etc. 

{c)  Oil  Substitutes  obtained  by  the  action  of  sulphur 
monochloride  on  vegetable  oils  to  reduce  the  specific  gravity 
of  low-grade  goods. 

[d)  Reclaimed  Rubber  to  cheapen  goods  where  the  addition 
of  heavy  mineral  matter  is  not  admissible. 

The  machine  in  which  the  mixing  is  carried  out  is  similar 
in  character  to  the  "  Washer,"  being  provided  with  hollow 
rollers  through  which  water  or  steam  can  be  passed.  Before 
the  actual  addition  of  the  ingredients,  the  rubber  is  first 
plasticized  "  by  passing  through  the  steam-heated  rollers. 


THE  RUBBER  HYDROCARBONS  23 


and  when  the  rubber  sheet  is  thin  enough,  the  other  materials 
are  gradually  added.  It  is  the  object  of  the  mixer  to  obtain 
an  homogeneous  dough  without  overworking  the  rubber. 
For  this  purpose,  the  dough,  while  still  hot,  is  passed  through 
a  calender  machine,  from  which  it  is  turned  out  in  the 
form  of  a  sheet,  and  led  to  a  revolving  wooden  roll  on  v/hich 
it  is  rolled  between  cloth.  The  calender consists 
essentially  of  superimposed  smooth  rollers,  two  or  more 
in  number,  between  which  the  rubber  can  be  fed.  The 
rollers  are  hollow  for  steam-heating. 

If  the  rubber  is  not  intended  to  be  prepared  in  sheet  form, 
the  material  from  the  mixing  rollers  may  be  passed  into 
moulds  or  forced  through  a  die,  as  when  solid  tyres  and  some 
forms  of  tubing  are  being  made. 

VUI^CANIZATION 

This  process  is  probably  the  most  important  that  rubber 
has  to  undergo,  and  without  it,  it  is  doubtful  whether  any 
rubber  industry,  worthy  of  its  name,  could  exist.  The 
process  consists  in  incorporating  with  the  rubber,  sulphur 
in  its  natural  form,  in  solution  or  in  combination  such  as 
poly  sulphides,  and  may  be  carried  out  by  the  hot  or  cold 
processes. 

Hot  Vulcanization  or  Hot  Cure. — This  process  was 
discovered  by  Goodyear  in  1839,  and  is  carried  out  by 
intimately  mixing  rubber  and  sulphur  by  mastication.  The 
process  is  facilitated  by  dissolving  the  rubber  in  naphtha 
and  then  mixing  in  sulphur.  The  temperature  is  raised  to 
100''  C.  At  this  temperature  chemical  union  takes  place 
between  the  constituents  of  the  mixture,  and  vulcanized 
rubber  is  formed.  The  whole  of  the  sulphur  does  not, 
however,  combine  with  the  rubber,  but  by  prolonging  the 
heating  at  lOo""  more  and  more  of  it  enters  into  combination 
and  a  darker  and  tougher  vulcanized  product  results.  The 
action  between  the  rubber  and  the  sulphur  does  not  start 
until  the  temperature  reaches  at  least  100°  C.  Various 
temperatures  ranging  from  i25°-30o''  C.  are  used,  depending 


24    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


on  the  form  in  which  the  sulphur  is  added,  but  for  proper 
vulcanization  it  is  essential  that  the  temperature  be  above 
the  melting  point  of  sulphur,  114°.  The  best  results  are 
obtained  when  high  pressure  is  used  in  conjunction  with 
high  temperature,  and  this  is  attained  either  by  wrapping 
tightly  with  fabric — e.g,  for  hose  and  tyres — or  by  using 
an  autoclave  and  injecting  live  steam. 

Cold  Vulcanization. — This  process  was  discovered  by 
Alexander  Parkes  in  1846,  and  is  used  more  particularly 
for  proofing  cloth  than  for  anything  else.  It  is  unsuitable 
for  thick  goods.  It  depends  on  the  action  of  sulphur  mono- 
chloride  on  rubber.  This  action  is  so  violent,  even  in  the 
cold,  that  the  sulphur  monochloride  has  to  be  diluted  by 
dissolving  in  carbon  disulphide  before  using.  The  process 
is  carried  out  thus  : — the  rubber  is  plasticized,  rolled  out  into 
a  sheet  and  dissolved  in  naphtha  with  which  it  forms  a  sort 
of  swollen  dough.  This  is  then  rendered  homogeneous  by 
being  worked  between  rolls  and  spread  upon  cloth  by 
passing  the  latter  between  four  series  of  rollers  and  a 
doctor,''  which  adjusts  the  thickness  of  the  rubber.  The 
solvent  is  removed  by  passing  over  a  steam-heated  table. 
The  rubber-covered  cloth  is  now  conveyed  to  another 
machine  upon  which  it  is  vulcanized  by  allowing  it  to  unwind 
from  one  roller  to  another  and  making  it  pass  over  an  inter- 
vening roller  which  dips  into  a  sulphur  monochloride  bath. 

The  rate  of  vulcanization  varies  with  the  temperature 
and  percentage  of  sulphur  present.  The  natural  proteids 
in  the  latex  favour  the  cure  as  their  removal  retards 
the  rate.  Inorganic  bases,  like  magnesia,  were  very  early 
catalysts  introduced  to  accelerate  the  hot  cure,  and  in  recent 
years  organic  bases  have  been  used  as  accelerators.  Piperidine 
was  one  of  these  which  was  first  patented,  but  later  the  best 
results  have  been  obtained  by  the  use  of  aldehyde-ammonia. 

Para-nitroso-dimethylaniline  is  another  effective  organic 
basic  accelerator,  the  time  for  curing  a  mixture  of  100 
parts  of  rubber  and  10  parts  of  sulphur  at  140"^  C.  being 
reduced  from  i  hour  to  20  minutes  when  O'S-O'S  of  this  base 
is  used  in  the  mixture.    If  this  substance  be  heated  with 


THE  RUBBER  HYDROCARBONS  25 


sulphur,  unknown  sulphur  compounds  are  formed,  which 
are  also  active  accelerators.  Twiss,  arguing  that  the  organic 
bases  are  active  because  of  their  solubility,  claims  that 
ordinary  caustic  potash  dissolved  in  glycerine  may  be  used 
instead  of  an  organic  accelerator  as  a  vulcanizing  catalyst. 

Peachey  uses  sulphuretted  hydrogen  and  sulphurous  acid 
gas  for  cold  vulcanizing. 

Artici.es  Manufactured 

Cut  Sheet. — This  is  used  for  tobacco  pouches,  surgical 
instruments,  etc.,  and  is  made  as  follows  :  Rubber,  after 
thorough  washing,  is  masticated,  and  when  homogeneous, 
is  made  into  a  solid  mass  and  frozen  hard.  The  mass  is  then 
sliced  by  a  knife  and  the  sheets  vulcanized  by  the  cold  process. 

Elastic  Thread,  for  braces,  garters,  spring-side  boots, 
etc.,  is  made  either  by  spreading  or  by  calendering.  In  the 
spreading  "  process  several  coatings  of  a  dough  consisting 
of  rubber  sulphur  and  naphtha  are  spread  on  to  sized  calico, 
and  after  dusting  with  French  chalk,  the  rubber  is  stripped 
from  the  cloth,  interlined  with  cloth  and  bound  round  a 
drum  which  is  then  vulcanized.  After  vulcanization,  the 
rubber  is  removed  from  the  drum,  pasted  over  with  a  solution 
of  shellac  in  methylated  spirit,  and,  while  still  wet,  wound 
round  a  roller  so  as  to  form  a  cylinder  which  dries  to  a  com- 
pact block.  The  rubber  is  then  placed  in  a  cutting  lathe 
and  the  thread  cut  out  to  the  required  gauge.  To  remove 
excess  of  sulphur  and  shellac,  the  threads  are  boiled  in  a 
solution  of  caustic  soda.  When  made  by  calendering,'' 
the  procedure  is  exactly  the  same,  only  one  starts  with 
calendered  sheet  instead  of  spread  sheet. 

Tyres. — Solid  Tyres,  such  as  cab  tyres,  omnibus  tyres, 
etc.,  are  made  by  squirting  an  appropriate  mixture  through 
a  die  in  a  forcing  machine.  The  dough  is  placed  in  a  hopper, 
and  by  the  motion  of  a  screw,  pushed  through  a  die,  from 
which  it  comes  out  as  a  long  thick  thread,  which  is  coated 
with  French  chalk  and  wound  on  a  tray.  It  is  finally 
vulcanized  by  the  hot  process. 

Pneumatic  Tyres. — The  inner  tubes  of  pneumatic  tyres 


26    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


are  usually  made  from  calendered  sheet  by  cutting  to  the 
desired  length,  lapping  round  a  mandril  and  joining  the  ends 
together  by  means  of  a  strip  of  rubber  and  rubber  solution. 
Good  results  are  also  obtained  by  squirting  as  for  solid 
tyres.  The  tubes  are  vulcanized  with  live  steam  on  the 
mandril,  and,  after  vulcanization,  removed,  and  the  ends 
joined  up  by  means  of  rubber  solution.  Vulcanization  is 
also  sometimes  done  in  moulds,  collapse  of  the  tube  being 
prevented  by  having  ammonia  or  ammonium  carbonate 
between  the  walls.  The  method  is  carried  out  in  the  case 
of  playing  balls  and  other  hollow  articles. 

Pneumatic  tyre  covers  are  made  up  on  a  mandril  from 
proofed  canvas  and  calendered  sheet  usually  by  hand  and 
vulcanized  in  moulds  in  open  steam. 

Hose. — This  is  prepared  from  calendered  sheet,  proofed 
canvas  and  other  materials  in  several  ways,  all  more  or  less 
variations  of  one  another.  Thus friction hose  is  made  by 
lapping  alternate  layers  of  calendered  sheet  and  canvas  round 
a  long  horizontal  mandril,  placed  in  a  wrapping  machine. 
The  same  result  may  be  obtained  by  drawing  a  seamless 
(squirted)  tube  through  a  mandril  and  then  lapping  over 
with  canvas,  or  by  passing  simultaneously  a  squirted  " 
tube  and  a  canvas  strip  through  a  machine  which  folds  the 
latter  over  the  tube. 

Buffers,  Valves,  etc. — Simple  buffers  are  made  by 
lapping  over  calendered  sheet  on  a  mandril  until  the  requisite 
thickness  is  obtained  and  then  vulcanizing  in  moulds.  The 
finished  article  is  sometimes  made  by  cutting  out  of  a 
vulcanized  cylinder. 

Belting. — This  is  made  by  spreading  out  a  suitable 
dough  on  both  sides  of  proofed  canvas  and  cutting  out 
into  strips.  The  strips  are  joined  together  until  a  suitable 
thickness  is  obtained  and  vulcanized  in  a  long  hydraulic  press. 

Heels  and  Rubber  Pads  are  made  by  stamping  out  of 
calendered  sheet  and  vulcanizing  in  moulds. 

Rings. — These  are  made  either  by  joining  together 
tape  or  cord,  or  by  cutting  from  a  tube  on  a  lathe. 

Ebonite  and  Vulcanite. — These  terms  are  applied 


THE  RUBBER  HYDROCARBONS  27 


to  the  product  obtained  when  rubber  is  vulcanized  for  a 
prolonged  time  (six  hours  instead  of  two)  with  a  very  large 
excess  of  sulphur  or  sulphur-containing  material  (25-40  per 
cent,  in  place  of  the  usual  5-10  per  cent.). 

The  material  is  very  hard,  relatively  non-elastic,  but 
capable  of  being  bent  without  breaking.  It  is  capable  of 
taking  a  high  polish,  may  be  turned  on  a  lathe,  softened  by 
heat,  moulded  and  pressed  out,  and  is  very  indifferent  to 
chemical  reagents  such  as  alkalis  and  acids.  For  this  reason 
it  finds  extensive  use  in  chemical  factories  for  pumps,  stop- 
cocks, and  proof  coverings,  accumulator  cases,  etc. 

It  is  prepared  in  a  manner  very  similar  to  ordinar}^ 
rubber  goods,  care  being  required  to  use  the  best  materials 
possible,  adding  the  foreign  substances  and  then  thoroughly 
mixing. 

Gutta-percha. — This  product  is  obtained  from  various 
East  Indian  trees  by  felling  and  ringing  the  bark  at  intervals 
of  I2~i8  ins.  A  latex  oozes  out  which  soon  coagulates. 
This  is  then  boiled,  washed  with  hot  water,  strained,  masti- 
cated between  rollers  and  sheeted.  Chemical  cleansing  with 
caustic  alkalis  or  bleaching  powder  is  sometimes  used,  and, 
when  used  for  making  a  certain  class  of  goods  such  as  golf 
balls,  etc.,  the  resin  which  is  present  is  extracted  by  treat- 
ment with  petroleum  spirit  which  leaves  the  gutta  unaffected. 

Gutta-percha  proper  is  closely  allied  to  caoutchouc,  the 
active  ingredient  of  ordinary  rubber,  but  it  is  not  identical 
with  it.  In  physical  properties  it  has  very  little  resemblance 
to  rubber.  It  has  not  the  same  elasticity,  on  warming 
becomes  very  plastic,  and  when  pressed  gives  even  the  finest 
relief  work  in  exact  detail. 

Under  ordinary  conditions,  gutta-percha  is  hard  but  not 
brittle,  forms  an  excellent  insulator,  and  for  this  reason 
finds  extensive  use  in  submarine  cables,  being  very  resistant 
to  high-water  pressures. 

Waste  Rubber. — A  very  large  quantity  of  scrap  rubber 
is  collected  annually.  The  greater  part  is  washed  and 
ground  up  finely  and  used  as  a  filler  in  low-grade  goods,  but 
the  rest  is  reclaimed  and  used  just  like  raw  rubber.  Various 


28    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


processes  are  employed  for  effecting  the  reclamation,  the 
most  important  being — 

1.  Washing  to  remove  all  dirt. 

2.  Treatment  with  alkalis  and  acids  to  remove  fibre  and 
metal  particles. 

3.  Treatment  with  thiosulphate,  sulphites,  etc.,  to 
remove  any  uncombined  sulphur  and  as  far  as  possible 
devulcanize  the  rubber. 

Rubber  Substitutes 

Owing  to  the  high  price  which  is  paid  for  rubber,  many 
attempts  have  been  made  to  find  substitutes  which  wotdd 
not  only  resemble  it  but  be  cheaper  to  produce.  The 
chief  of  these  substitutes  is  that  obtained  from  drying  oils 
such  as  linseed,  which  is,  of  course,  known  to  give  an 
elastic  film  when  it  dries.'' 

When  linseed  oil  is  treated  with  sulphur  (sulphur  mono- 
chloride)  or  antimony  sulphide,  an  elastic  mass  is  produced 
which  varies  in  colour  according  to  the  process  used.  These 
elastic  masses  are  used  as  cheapening  additions  to  raw 
rubber. 

Other  substitutes  are  obtained  by  heating  together  rape 
oil  with  sulphur,  and  castor  oil  with  sulphur  monochloride. 

Mixtures  of  glue,  gelatine,  glycerine,  oils,  etc.,  after 
treatment  with  tannic  acid,  chromates,  formaldehyde,  have 
been  used  as  substitutes  for  rubber. 

In  spite  of  all  the  work  that  has  been  done,  no  rubber 
substitute  which  can  be  used  by  itself,  i,e,  without  any 
addition  of  rubber,  is  on  the  market. 

Seeds  and  their  O11.S 

A  mature  Hevea  tree  will  produce  on  an  average  five 
hundred  seeds  per  year,  and  as  all  these  cannot  under  any 
circumstances  be  planted  it  is  evident  that  a  very  large 
quantity  of  seeds  must  become  available  for  commercial 
purposes  every  year. 


THE  RUBBER  HYDROCARBONS 


29 


The  seeds  must  be  collected,  decorticated  and  dried 
before  being  transported,  so  that  the  question  arises  as  to 
the  commercial  value  of  the  oils  from  the  seeds  and  the 
meal  cake  produced  after  the  oil  is  pressed  out  from  them. 
The  collection  is  to  a  certain  extent  compulsory,  as  under- 
growth and  rodent  life  must  be  reduced  to  a  minimum  ; 
decortication  is  necessary,  as  there  is  no  other  use  for  the 
shells,  which  weigh  more  than  half  the  kernel,  other  than 
locally  as  fuel.  Hence  the  value  of  the  seed  depends 
entirely  on  its  oil  and  oil  cake. 

The  fresh  seed  consists  of  about  one-third  shell  and  two- 
thirds  kernel. 

Hevea  seed  cake  after  expression  of  oil 
Composition  of  cake  (kernel).  (kernel  only). 


Moisture 

•  Q-i 

Moisture 

•  13-36 

Ash 

•  3-53 

Ash 

•  5-19 

Fibre    . . 

•  3-4 

Fibre 

5-00 

Oil 

.  36-1 

Oil 

6-00 

Proteins 

.  i8-2 

Proteins 

.  26-81 

Carbohydrates 

2Q"67 

Carbohydrates  . 

•  43-64 

100*00 

lOO'OO 

It  is  evident,  therefore,  that  the  kernels  contain  about 
one-third  their  weight  of  oil. 

The  oil  is  clear,  light  yellow  in  colour,  and  on  saponifica- 
tion with  soda  yields  a  soft  soap.  It  has  been  stated  that 
it  can  be  used  for  the  preparation  of  varnishes,  and  further 
investigations  have  shown  that  it  is  suitable  as  a  substitute 
for  linseed  and  similar  drying  oils.  lyittle  seems  to  have 
been  done  on  the  lines  of  utilizing  this  oil,  owing  probably 
to  high  cost  of  seed  collection  and  the  fact  that  it  would 
have  to  compete  with  other  oils  on  the  market ;  but,  given 
local  seed  pressing  machinery,  there  seems  to  be  no  reason 
why  this  source  of  wealth  should  remain  undeveloped.  See 
further,  p.  56. 

Gear  a  Rubber  Seeds. — These  are  small  hard  seeds 
weighing  about  53  gms.  per  100.  They  are  difiicult  to  shell, 
mincing  processes  giving  only  45  per  cent,  kernel.  The  shell  so 
obtained  is  unsuitable  for  cattle  feeding  unless  finely  ground. 


30    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


The  oil  amounts  to  35  per  cent,  of  the  kernel,  but  only 
15  per  cent,  on  the  whole  seed.  It  has  a  light  yellow  colour, 
agreeable  odour  and  taste  and  resembles  linseed  oil  in  having 
a  high  iodine  value,  drying  quickly  and  giving  a  tough  elastic 
skin  very  white  in  colour.  On  boiling  it  becomes  very 
viscous  and  forms  a  transparent  gelatinous  mass  when  treated 
with  sulphur  chloride. 

Funtumia  elastica. — These  are  small  pointed  seeds 
with  husks  thin  and  soft.  One  hundred  seeds  weigh  about 
5  grams.  The  oil  is  very  dark  in  colour  and  amounts  to 
33  per  cent,  on  the  seed.  Ordinary  decolorizing  agents 
have  little  effect  on  it.  It  froths  on  boiling  and  has  a  bitter 
odour  and  taste,  and  deposits  mucilaginous  matter  accom- 
panied by  stearine  in  cold  weather.  It  has  the  properties 
of  a  drying  oil,  but  gives  a  tacky  film.  On  heating  it  darkens 
in  colour,  and  forms  a  viscous  liquid  similar  to  linseed  oil 
freshly  extracted.  Treatment  with  sulphur  chloride  gave 
an  elastic  but  sticky  substance. 

The  physical  and  chemical  properties  of  these  oils  com- 
pared with  linseed  and  Hevea  oils  are  given  below : — 


Ceara. 

Funtumia. 

Hevea. 

Linseed. 

Weight  of  100  seeds    . . 

53*2 

4-8 

360  gms. 

Oil  content  of  kernel    . , 

35-o% 

45-48% 

„       „    of  total  seed 

1575% 

3 1  •0-33-0% 

22-25% 

36-40  7o 

Sp.gr  

0-9238 

0-9320 

0-9258 

0-93I--938 

Refractive  index  (15®  C.) 

1*475 

1-4788 

1*4835 

Relative  viscosity 

130 

14*3 

i8-o 

Iodine  value 

i35-o-i37'o 

138-0 

1387 

l80°-200° 

Saponification  value    . . 

189-1 

185-0 

191*2 

190-195 

Acid  value 

0625 

2-65 

5-26 

below  5 

Hehner  value    . . 

95'i 

94-0 

94-81-95-5 

Reichert-Meissl 

0-44 

0-66 

2-7 

o-oo 

Liquid  fatty  acids 

88-9% 

79-8% 

86-o% 

92-5% 

Iodine  value  thereof    . . 

162-5 

175-5 

I90°-2IO° 

Fat-Free  Residue 


Per  cent. 

Per  cent. 

Nitrogen 

2*19 

4*34 

Proteins 

.  .  14-23 

27-08 

Ash  

.  .  17-10 

5-04 

Potash 

2-65 

1-44 

Phosphoric  acid 

1-72 

2-25 

Sand 

0-72 

0-32 

THE  RUBBER  HYDROCARBONS  31 


Diseases  and  Pests 

The  fungoid  diseases  of  Hevea  have  been  carefully 
studied  both  in  Ceylon  and  in  Malaya,  and  many  of  them 
seem  to  be  identical.  From  time  to  time  certain  of  these 
diseases  become  more  virulent  and  attract  greater  attention, 
but  on  most  estates  the  treatment  is  known,  and  a  sharp 
look-out  is  kept  which  makes  the  mortality  from  fungoid 
diseases  not  a  serious  factor.  The  root  diseases  are  those 
which  are  chiefly  to  be  feared,  as  in  many  of  them  the 
mycelium  travels  through  the  soil  and  thus  spreads  the 
disease  from  tree  to  tree.  Isolation  by  digging  a  trench 
round  the  tree  and  the  excision  of  all  the  infected  roots 
with  subsequent  burning  is  the  treatment  in  the  early 
stages.  The  roots  of  neighbouring  trees  should,  however, 
be  examined  to  be  sure  that  the  fungus  has  not  reached  them. 
The  common  root  diseases  are  due  to  Fomes  lignosus 
(formerly,  semitostus),  Ustulina  zonula  (which  originally 
was  studied  in  its  ravages  on  tea).  Porta  hypolateria  (known 
as  hypobrunnea  in  Ceylon),  Sphaerostilbe  repens  and  Hymeno- 
chaete  nexia  or  brown  root  disease.  Diseases  alfecting  the 
bark  and  leaf  surface  are  more  easily  detected.  In  districts 
with  a  heavy  rainfall.  Pink  disease,  caused  by  Corticium 
Salmonicolon,  appears  on  the  underside  of  the  bark  of  the 
stem  and  branches  of  the  Hevea.  This  fungus  also  grows 
on  most  tropical  plants  and  has  caused  trouble  on  coffee, 
tea,  cinchona  and  other  plantations. 


Synthetic  Rubber 

Synthetic  rubbers  have  been  prepared  in  the  laboratory 
by  the  polymerization  of  unsaturated  hydrocarbons.  This 
change  is  hastened  by  catalysts.  As  far  back  as  i860, 
Greville  Williams  noticed  that  isoprene  which  he  obtained 
by  distilling  rubber,  reverted  back  into  a  viscous  solid  on 
keeping,  and  in  1879  Bouchardat  found  that  in  presence  of 
hydrochloric  acid,  the  same  condensation  took  place  more 


32    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


rapidl}^  Piperylene,  an  isomer  of  isoprene,  behaves 
similarly.  Tilden  found  that  nitrosyl  chloride  also  acted 
as  a  catalyst  in  polymerizing  isoprene  to  rubber,  and  in 
1892  showed  that  this  synthetic  rubber  could  be  vulcanized. 
About  the  same  time,  Couturier  proved  that  dipropylene 
similarly  polymerized  by  heat  to  a  solid  rubber,  and 
Matthews,  in  1910,  patented  the  use  of  sodium  for  bringing 
about  the  polymerization  of  butadiene  (erythrene)  and  its 
homologues.  In  this  way,  20  parts  of  isoprene,  mixed 
with  I  part  of  sodium,  in  the  cold  becomes  solid,  and  any 
unchanged  liquid  can  be  distilled  oS.  or  extracted  by  a  suit- 
able solvent.  Harries  confirmed  this  useful  catalytic 
property  of  sodium  on  these  unsaturated  hydrocarbons, 
and  more  recently,  acetic  anhydride  at  150°  has  been 
patented  for  the  conversion  of  isoprene  to  rubber,  and 
even  small  quantities  of  acid  or  sulphur  promote  the  con- 
densation when  heated  with  the  hydrocarbon.  Oxygen 
carriers,  sodium  or  zinc  ethyl  and  colloidal  metals  are 
also  the  subject  of  other  processes  for  bringing  about  this 
change. 

Ostromyslenski,  in  Russia,  during  the  war  suggested 
that  isoprene  could  be  catalytically  converted  first  into 
^-myrcene,  and  that  this  new  hydrocarbon  could  be  finally 
condensed  to  caoutchouc,  having  all  the  properties  of  Para 
rubber. 

Much  further  study  is  required  as  to  the  yields  obtained 
by  these  various  processes  before  one  can  predict  which  is 
likely  to  be  economically  successful. 

The  source  of  the  parent  isoprene  or  other  unsaturated 
hydrocarbon  and  cost  price  of  the  same  has  also  to  be  con- 
sidered before  one  can  arrive  at  any  measure  of  the  future 
possible  competition  between  synthetic  caoutchouc  and  that 
obtained  from  plantations.  These  parent  hydrocarbons  are 
now  obtainable  by  the  removal  of  water  by  passing  ketones 
or  alcohols  over  suitable  dehydrating  catalysts.  Thus, 
starting  with  dihydroxymethyl  butane  one  can  obtain  the 
corresponding  ketone  and  finally  isoprene  by  using,  accord- 
ing to  one  patent,  aluminium  silicate  as  the  catalj^st  at 


THE  RUBBER  HYDROCARBONS  33 


a  temperature  between  400°  and  600''  under  diminished 
pressure. 

CH20H.CH(CH3)  .CHOH.CH3  -2OH2  -CH2:C(CH3)  .CHiCHs 

If  an  ester  like  the  acetate  of  methyl  butenol  be  heated  with 
alumina  under  diminished  pressure  to  400°,  acetic  acid  and 
isoprene  are  formed. 

CH3.C(CH3).(OCOCH3).CH  :  CH. 

=CH3.COOH+CH2  :  C(CH3).CH  :  CH2 

Ostromyslenski  starts  from  acetaldehyde  and  an  alcohol 
with  a  catalytic  dehydrating  agent,  such  as  precipitated 
alumina  which  owes  its  activity  to  traces  of  basic  salts, 
thus — 

CH3.CHO+CH3.CH2OH-2H2O+CH2  :  CH.CH  :  CHo 

Erythrene. 

CH3.CHO  +  (CH3)  2CHOH  =2H20  +CH(CH3)  :CH.CH:CH2 

Piperylene. 

The  best  yields  seem  to  be  not  more  than  16-18  per  cent,  of 
the  pure  hydrocarbon. 

These  syntheses  of  the  butadienes  thus  put  the  parent 
material  a  stage  further  back  to  the  alcohol  or  ketone,  and 
it  is  still  doubtful  whether  these  can  be  obtained  com- 
mercially say,  from  starch,  by  fermentation  or  by  a  synthesis 
from  carbide  at  such  yields  and  at  a  price  which  would  enable 
an  industrial  development  to  follow. 


BIBLIOGRAPHY. 

Torrey  and  Manders,     The  Rubber  Industry.''    London.  1914- 
Herbert  Wright,  "  Para  Rubber."    London.  1912. 
Sidney  Morgan,  "The  Preparation  of  Plantation  Rubber."  London.  1913* 
Cuthbert  Christy,  "  African  Rubber  Industry."    London.    191 1. 
Hinrichsen,  U.  Memmler,  "  Der  Kautschuk  und  seine  Prufung." 
Weber,  C.  O..  **  The  Chemistry  of  Indiarubber."    London.  1902. 
R.  Ditmar,  "  Die  Analyse  des  Kautschuks,  etc."    Wien  and  Leipzig. 
1909. 

Schidrowitz,  P.,  "  Rubber."    London.    191 1. 
Potts,  H.  E.,  "  The  Chemistry  of  the  Rubber  Industry." 
Lewis,  E.  W.,  "  Aliens'  Commercial  Organic  Analysis,"  Vol.  II. 
Pellier,  P.,  "  Guide  de  I'acheteur  de  Caoutchouc  Manufacture." 
C^spari,  W.  A.,  "  Indiarubber  Laboratory  Practice."    London.  1914. 
Paris.  1912. 

Herbst,  E.,  "  Kautschuk,"  in  Pott's  Chemische  Technische  Analyse. 
Bard  II.    Braunschweig.  1909. 

s  3 


34   RUBBER.  RESINS,  PAINTS  AND  VARNISHES 


H.  C.  Pearson,  "  Crude  Rubber  and  Compounding  Ingredients."  New 
York.  1910. 

Heil  and  Esch,  "  The  Manufacture  of  Rubber  Goods."    London.  1909. 

H.  L.  Terry,  '*  Indiarubber  and  its  Manufacture."    London.  1907. 

R.  Ditmar,  "  Die  Analyse  des  Kautschuks  der  Gutta-percha,"  Balata, 
und  ihrer  zusatze  mit  einschhiss  der  Chemie  der  Gerannten  Stoffe  Wien  and 
Leipzig.  1909. 

R.  Ditmar,  "  Kautschuk,"  Dammer's  Chemische  Technologie  der 
Neuzeit.    Bd.  III.,  p.  631.    191 1. 

Rideal  and  Acland,  *'  Other  Rubber  Seed  Oils,"  Analyst,  1913,  259. 

Ostromyslenski,  /.  Russ.  Phys.  Chem.  Soc,  191 5  :  471928-31,  1441  ; 
1915:  47  et  seq.;  1910  et  seq.;  1910 :  1472-1494,  1947-1982,  1983-1988  : 
1494-1506;  Ber.  1914:  472350-354- 

C.  Harries,  "  Liebigs  Annalen."  191 1,  383  ;  "  Gummi  Zeit."  1910,  850  ; 
1907:  21,  823;  1906:  1277;  ^eits.  angew,  Chem.,  191330;  Ber. 
1904:  372708;  1905:  381195,  1198,  3985,  3989. 

Kondakow.  Journ.  Pr.  Ch.,  1899  (2):  59299;  1900;  62175;  1901  : 
631 13  ;  1901  :  64109,  no  ;  Revue  Generale  de  Chimie,  191 2. 

Perkin,  W.  H.,  Jun.,  "  Butadiene,  Isoprene  and  their  Homologues," 
J.S.C.I.,  1912,  616. 

Tilden,  Chem.  News,  1882:  46120;  1892:  65265;  /.  C.  Society.  1884  i 
4541 1  ;  Brit.  Assoc.,  1906. 

Willstatter,  Ber.  38.    1975  ;  40,  957,  3994. 

Klages,  "  Kautschuk  u.  Kautschukersatzstoffe."  Zeits.  angew.  Chem,, 
1911  :  34,  1505. 

Patents  Bearing  on  the  Manufacture  of  Synthetic  Rubber. 

English  Patents.    1907:  21772  ;  1908:  13252  ;  1909:  15299,  29277,  29566  ; 

1910 :  4001,  4189,  4572,  4620,  5931,  5932,  8100,  9219,  14040,  14041, 

15254,  17734,24790,25850;  1911:  1124,  1125,  6540,  9721,  9722,  15203, 

15204,  16925,  18935,  27361. 
French  Patents.    1911 :  434586,  433825,  433322;   1912 :  438789,  277, 

448,  3873.  5555>  12770,  12771,  12772,'  12773,  13051  ;  1914:  47565. 
German  Patents.    1913  :  286640  ;  1914  :  279780. 


Part  II.— DRYING  OILS 

The  class  of  oils  which  the  paint  and  varnish  maker  uses  show 
the  property  of  drying  "  in  the  air.  The  drying  of  a  film 
of  paint  or  varnish  depends  primarily  on  the  nature  of  the 
vehicle.  The  difference  can  be  shown  by  an  experiment 
whereby  plates  of  glass  are  coated  respectively  with  water, 
petrol,  kerosene  or  paraffin  and  linseed  oil.  After  several 
days'  exposure  the  petrol  and  water  will  have  evaporated, 
while  the  plate  coated  with  kerosene  will  be  found  to  be 
greasy  and  almost  unchanged.  The  linseed  oil  coating  will 
have  become  tacky  and  will  finally  set  to  a  tough  varnish- 
like film.  lyinseed  oil  is  a  typical  drying  oil.  Some  oils, 
e.g,  olive  oil  and  castor  oil,  will  behave  like  the  paraffin 
layer  and  are  non-drying  oils.  Semi-drying  oils  will  become 
tacky  very  slowly  and  may,  on  prolonged  exposure,  yield 
flexible  films.  This  property  of  drying  can  be  accelerated 
by  incorporation  of  driers,"  to  which  fuller  reference  will 
be  made  later.  In  spite  of  the  vast  quantities  of  linseed 
and  other  drying  oils  handled  annually  the  changes  occurring 
during  the  drying are  not  fully  understood.  Un- 
doubtedly linseed  oil  is  the  most  important  member  of  its 
class,  and  its  general  properties  may  be  considered  typical. 
It  contains  the  gly cerides  of  unsaturated  acids  of  the  aliphatic 
series,  with  i8  carbon  atoms  in  an  open  chain.  It  is  of 
interest  to  record  that  no  drying  oil  of  vegetable  origin  is 
known  derived  from  an  acid  containing  less  than  i8  carbon 
atoms  in  the  molecule  (isanic  acid  C14H20O2  is  perhaps 
an  exception).  The  majority  are  open-chain  compounds 
with  the  exception  of  the  oils  from  cliaulmoogra  seeds 
(Barrowcliff  and  Power,  /.  Chem.  Soc,  1907,  loi,  577). 
As  gly  cerides  they  are  saponifiable  and  are     fatty    oils  in 

35 


36   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 

contradistinction  to  mineral  oik,  which  are  unsaponifiable. 
They  may  also  be  termed  fixed  oils,  in  that  they  cannot  be 
volatilized  under  atmospheric  pressure  without  decom- 
position. The  drying  oils  of  animal  origin  (menhaden  and 
Japanese  fish  oil)  contain  the  glyceride  of  clupanodonic  acid, 
C18H28O2,  whilst  the  liver  oils  of  certain  members  of  the 
shark  family  contains  spinacene,  a  highly  unsaturated 
hydrocarbon  (Chapman,  /.  Chem,  Soc,  1917,  iii,  56).  It 
is  possible  that  this  substance  is  identical  with  squalene,  also 
prCvSent  in  shark  oil  (Tsujimoto,  /.  Ind.  Eng.  Chem.,  1916,  S, 

889). 

One  characteristic  difierence  between  animal  and  vege- 
table oils,  including  drying  oils  of  animal  and  vegetable 
origin,  lies  primarily  in  the  presence  of  cholesterol,  C27H45OH 
(an  alcohol),  which  is  a  component  of  the  well-known  lanolin ; 
whereas  vegetable  oils  contain  phytosterol  (an  alcohol  of 
the  same  formula),  which  is  distinguishable  under  the 
microscope  and  yields  an  acetate  with  a  melting  point 
different  to  that  obtained  from  cholesterol. 

Genuine  linseed  oil  is  essentially  a  mixture  of  the  tri- 
glycerides of  linolenic,  linolic,  and  oleic  acids,  together  with 
small  quantities  of  glycerides  of  saturated  aliphatic  acids, 
palmitic,  stearic,  and  possibly  myristic  acids.  The  glycerides 
are  mixed  glycerides  of  varying  composition  depending  on 
the  source  of  origin  and  on  the  maturity  of  the  seed  from 
which  the  oil  has  been  expressed.  The  amount  of  saturated 
glycerides  is  small,  but  their  presence  in  linseed  oil  is  a  factor 
for  consideration  in  any  varnish  process. 

Friend,  "  Chemistry  of  Linseed  Oil,"  1917,  p.  64,  sum- 
marizing the  evidence  up  to  date,  states  the  percentage 
composition  as  follows  : — 


Saturated  organic  acids  . . 

. .  lo-o 

10  "0 

Oleic  acid  . . 

5-0 

5-0 

lyinolic  acid 

48-3 

59-1 

lyinolenic  acids    . . 

. .  32-1 

21-3 

Glyceryl  radicle  (C3H5) 

..  4-6 

4-6 

Total 

. .  100 -0 

100 -0 

DRYING  OILS 


37 


Fahrion  (Z.  angew.  Ckemie,,  1910,  2j,  722  and  1106) 
gives  a  slightly  different  composition  : — 

Unsaponifiable  matter   . .        . .      0*6     per  cent. 
Saturated  fatty  acids 


Oleic  acid  . . 
lyinolic  acid 
lyinolenic  acids 


8-6 
15-20 
30 
38 


The  most  important  component  of  linseed  oil  is  the 
glyceride  of  linolenic  acid,  C17H29COOH,  or 
CH3CH2.CH-CH.CH2.CH=CH.CH2.CH-CH(CH2)7COOH 
(Goldsobel,  /.  Russ.  Phys.  Chem,  Soc,  1906,  j<?,  1904 ; 
1910,  42,  55  ;  Erdmann  and  Raspe,  Ber,;  1909,  42,  1334  ; 
Erdmann  and  Bedford,  Ber,,  1909,  42,  1324.) 

Stearic  acid  is  a  saturated  acid  possessing  the  formula 
CH3(CH2)i6COOH. 

Linolenic  acid  is  stated  to  occur  in  two  forms  in  linseed  oil, 
as  glycerides  of  a  and  ^-linolenic  acid  (Erdmann  and  Bedford, 
loc,  cit,).  From  the  work  of  Erdmann  and  his  collaborators 
linolenic  acid  maybe  taken  as  containing  three  double  linkages 
as  shown  above.  From  the  general  properties  of  such  a  con- 
figuration it  is  to  be  expected  that  the  oxidation  of  linolenic 
acid  will  proceed  in  stages,  whereby  a  molecule  of  oxygen 
becomes  attached  where  there  is  a  double  linkage.  This 
gradual  absorption  has  been  observed  in  the  oxidation  of 
China  wood  oil  (/.  Chem.  Soc,  1918,  iij,  iii).  Peroxides 
O— CH 

of  the  type   |  are  formed   (Fahrion,   Chem,  Zeit.y 

O— CH 

1904,  28,  1196  ;  Wilson  and  Heaven,  /.  5.  C.  /.,  I9i2,j/  , 
565  ;  and  Ingle,  /.  5.  C.  /.,  1913,  J2,  639),  and  eventually  from 
the  linolenic  acids  there  result  glycerides  of  diperoxylinolenic 
acids  when  linseed  oil  is  transformed  into  the  highly  oxidized 
state  of  linoleum  (A.  de  Waele,  /.  Ind,  Eng,  Chem.,  1917, 

*  Cf.  Engler  and  Frankenstein,  Ber.,  1901,  34,  2933,  on  the  oxidation  of 
dimethyl  fulvene: — 

CH— CH  O2 
CH=CHv  /CHo  I    \/\  /CH2 

I  yC  =  C<  {Dimethyl  +  202  = 

CH^CH-^        ^CHs  ^^^^^^^> 


38   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 

The  relative  positions  of  the  peroxide  attachments  is 
not  definitely  fixed,  but  it  may  be  presumed  that  they  are 
contiguous  to  the  (CH2)7COOH  group  end  of  the  chain. 
Ingle  (/.  S.  C.  1902,  21,  594)  states  that  the  doubly 
linked  carbon  atoms  in  proximity  to  CO  OH  groups  may  be 
prevented  from  absorbing  halogens  from  a  strongly  acid 
Hubl  solution.  Such  peroxides  may  undergo  subsequent 
polymerization  (Fahrion,  Chem,  Zeit.,  1904,  1196,  and 
Morrell,  loc.  cit.)  which  may  occur  by  linkage  up  of  the 
molecules  through  unattacked,  unsaturated  carbon  atoms, 
rather  than  through  the  peroxide  groups : — 

O— CH— CX— CY— CH— O 

I     I       I       I       I  I 
O— CH— CX— CY— CH— O 

lyinking  up  through  unsaturated  carbon  atoms  is  more 
likely  because  of  the  probable  occurrence  of  the  glyceride 
of  diperoxylinolenic  acid  in  linoleum  {loc.  cit.)  rather  than 
a  triperoxy-acid,  and  also  the  subsequent  polymeriza- 
tion of  monoperoxy-a-elaeostearic  acid  obtained  from 
tung  oil.  It  must  be  pointed  out  that  these  peroxides 
are  unstable  and  their  possible  products  will  be  discussed 
later. 

Fokin  (/.  Russ.  Phys.  Chem.  Soc,  1908,  ^0,  276)  considers  » 
that  the  primary  product  in  the  oxidation  is  an  oxide 
R.CH— CHR 

\/.         General  evidence  favours  a  peroxide  formula 
O 

for  the  oxidation  product  of  linseed  oil,  but  the  subsequent 
change  to  linoxyn  may  be  accompanied  by  polymerization 
or  by  decomposition  of  the  oxidized  molecules  with  poly- 
merization of  the  products  (Harries,  Annalen,  1906,  j^j,  318) ; 
and  A.  de  Waele,  Chem.  World,  1914,  j,  300). 

Salway  (/.  Chem.  Soc,  1916,  lop,  138)  suggests  that 
linoxyn  consists  of  olein  and  of  polymerized  aldehydes 
derived  from  the  decomposition  of  the  oxidized  linolenin. 
De  Waele  considers  it  to  consist  of  peroxides  with  poly- 
merized aldehydoglycerides  {loc.  cit.).   Orloff  (/.  Russ.  Phys. 


DRYING  OILS  39 

Chem.  Soc.y  1910,  658)  suggests  that  linolenic  and  linolic 
glycerides  on  oxidation  yield  respectively  : — 

CH3.CH2.CH— CH.CH2.CH— CH.CH2.CH-CH(CH2)7.COOR 

\/  \/  \/ 

O  O2  o 

CH3.CH2.CH— CH.CH2.CH-^CH(CH.)ioCOOR 

o  o 

These  formulae  do  not  account  for  the  presence  of  unsaturated 
components  of  linoxyn.    [See  also    Theory  of  Driers/'] 

Without  going  further  into  the  undecided  changes  in  the 
oxidized  molecule,  it  is  sufficient  to  state  that  the  tough 
elastic  film  obtained  when  linseed  oil  is  oxidized  is  linoxyn, 
and  that  it  contains  the  glyceride  of  diperoxylinolenic  acid 
with  possibly  some  peroxylinolic  acid  glyceride,  together 
with  their  decomposition  products. 

The  systematic  examination  of  the  oxidation  procucts 
of  linseed  oil  is  comparatively  recent,  in  spite  of  the  great 
antiquity  of  its  employment  in  the  arts  and  crafts. 

Th^  conditions  of  absorption  of  oxygen  will  be  discussed 
under  the  theory  of  driers,  but  it  will  be  of  advantage  to 
study  briefly  the  reactions  of  unsaturated  glycerides,  so  as 
to  understand  the  behaviour  of  drying  oils.  The  researches 
of  Harries  and  his  pupils  {Ber.^  1906,  jp,  2894,  and  1909,  ^2, 
442),  Molinari  {Ber.,  1906,  jp,  2735,  and  1908,  2794), 
Erdmann  and  Raspe  {loc.  cit,),  and  Fenaroli  {Gazz,  chim. 
itaL,  1906,  j6,  (2),  292)  on  the  action  of  ozone  on  unsaturated 
acids  and  oils  have  shown  that  a  molecule  of  ozone  can  be 
attached  to  each  pair  of  doubly  linked  carbon  atoms.  Such 
ozonides,  e,g.  oleic  acid-ozonide 

CH3(CHo)..CH— CH— (CHc>)7C00H 
O3 


are  easily  decomposed  by  water  and  by  alkalies  to  give  acids 
and  aldehydes  according  to  the  schema — 


40    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


R.CH— CH.R'.COOH  R.COOH+CHO.R'.COOH 

Acid.  Aldehydo  acid. 

COOH 

^    RCOOH+R'  ^^Sdf 
COOH 

Tlie  identification  of  the  products  of  decomposition  has 
thrown  light  on  the  structure  of  the  unsaturated  acids,  and 
Erdmann  and  Raspe's  formula  for  linolenic  acid  rests  on 
the  results  of  such  an  investigation.  For  details  as  to  the 
properties  of  the  ozonides  and  their  importance  in  fixing  the 
structure  of  linseed  and  other  oils,  reference  ma}^  be  made  to 
Friend's  monograph  on  the  Chemistry  of  Linseed  Oil/' 
1917,  pp.  18,  44,  which  is  supplied  with  an  excellent 
bibliography  on  the  subject. 

In  addition  to  ozone,  hydrogen  may  be  made  to  unite 
with  glycerides  of  the  unsaturated  oil  acids  by  modifica- 
tions of  Sabatier  and  Senderens'  method,  whereby  hydrogen 
is  absorbed  in  the  presence  of  nickel  as  catalyst,  with  the 
formation  of  a  saturated  acid  or  its  glyceride,  viz.  stearic 
acid 

X— CH  -CH.  Y  +2H  -X.CH2— CH2.  Y 

The  hardening  of  oils,  whereby  the  liquid  fat  is  trans- 
formed into  solid  stearin,  is  now  an  important  industry 
which  may  be  said  to  date  from  the  time  when  the  scientific 
investigation  of  oils  was  systematically  •undertaken.  The 
reduction  is  quantitative,  and  Bedford  {loc.  cit)  has  demon- 
strated that  the  hydrogen  absorbed  can  be  taken  as  a 
measure  of  the  degree  of  unsaturation  of  the  oil ;  this  is 
usually  decided  by  the  determination  of  the  iodine  value, 
which  has  now  become  a  recognized  method  for  the  examina- 
tion of  drying  oils. 

X— CH  =CH— Y+2I  -X— CHI— CHI.  Y 

The  iodine  is  usually  presented  in  the  form  of  ICl  and  the 
excess  determined  volumetrically  (Fryer  and  Weston,  Oils, 
Fats,  and  Waxes,''  1918,  2,  94).  Sufficient  has  been  given  to 
indicate  the  unsaturated  character  of  linseed  oil  as  a  typical 


DRYING  OILS 


41 


drying  oil  as  shown  by  its  power  to  absorb  elements  such  as 
oxygen,  h}' drogen,  chlorine,  bromine  and  iodine. 

If  a  layer  of  linseed  oil  (o*i-0'2  gram  per  100  sq.  cms.) 
be  spread  on  glass,  the  maximum  gain  in  weight  is  19  per 
cent,  of  the  weight  of  the  oil  taken  (lyippert,  Z.  angew,  Chem., 
1898,  II,  412  ;  Weber,  ibid.,  508).  The  rate  of  absorption 
depends  on  the  temperature,  atmospheric  conditions  and  on 
the  presence  of  catalysts  or  driers.''  The  absorption  is 
accompanied  by  a  decomposition  due  to  the  degradation  of 
the  peroxides  previously  referred  to.  The  decomposition 
products  comprise  volatile  substances,  e.g,  carbon  dioxide, 
water,  formic  and  acetic  acids,  aldehydes  (acrolein,  C3H4O) 
(Salway,  Trans.  Chem,  Soc,  1916,  lop,  136),  caused  by  the 
disruption  of  the  molecule  where  the  oxygen  has  been 
attached  to  the  double  linkages.  Such  aldehydes  may  have 
a  bactericidal  action,  and  if  present  in  large  concentration 
they  are  considered  by  some  to  have  toxic  properties.* 

Moisture  and  soluble  ferments  bring  about  hydrolysis  of 
the  glycerides  whereby  acids  are  formed,  which  on  oxidation 
and  rupture  of  the  molecule  produce  aldehydes  causing 
rancidity  "  (Nicolet  and  lyiddle,  /.  Ind.  Eng.  Chem.,  1916, 
c?,  416)  . 

Owing  to  the  loss  of  volatile  products  the  percentage  of 
oxygen  absorbed  after  56  days'  exposure  may  be  2*25  times 
the  observed  increase  in  weight  of  the  oil,  if  it  is  allowed 
free  exposure  to  air  and  no  film  is  allowed  to  form. 

(1)  Time  of  exposure,  56  days ;  increase  in  weight  of  the 
oil,  974  per  cent. ;  weight  of  volatile  products,  12*2  per 
cent. ;  weight  of  oxygen  absorbed,  21-49  per  cent. 

(2)  Time  of  exposure,  68  days ;  increase  in  weight  of  the 

*  It  is  significant  that  no  evidence  has  been  shown  of  the  toxicity  of  the 
gases  evolved  during  the  oxidation  of  linseed  oil  in  enclosed  spaces,  such 
as  are  found  in  the  "  oxidizing  sheds  "  in  linoleum  works  (see  chapter 
on  Linoleum).  The  concentration  of  the  vapour  evolved  in  these  chambers 
where  solidification  of  linseed  oil  at  the  rate  of  one  ton  per  diem  takes 
place,  is  such  that  their  irritant  effect  on  the  mucous  membrance  of  the  nose, 
and  the  lachrymatory  glands  of  the  eyes,  make  it  impossible  for  any  but 
operatives  inured  to  the  vapours  to  withstand  them  with  comfort  for  more 
than  a  minute  or  so.  Workmen  employed  in  the  oxidizing  sheds  are  said 
to  remain  in  them  for  periods  of  an  hour  at  a  time,  without  suffering  any 
ill  effects^  other  than  a  passing  discomfort  due  to  hypersecretion  of  the 
glands  mentioned. 


42    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


oil,  8*96  per  cent. ;  weight  of  volatile  products,  15-8  per 
cent.  ;  weight  of  oxygen  absorbed,  2477  per  cent.  (Friend, 
Proc.  Paint  and  Varnish  Soc,  May  14,  1914  ;  Wilson  and 
Heaven,  /.  5.  C.  /.,  1912,  j/,  565 ;  and  de  Waele,  Chem. 
World,  1914.) 


Fig.  I. — Percentage  of  Oxygen  absorbed  (Wilson  and  Heaven). 


From  the  above  data  there  is  a  point  of  equilibrium  at 
which  the  gain  in  weight  is  approximately  equal  to  the  loss 


Fig.  2. — Percentage  Gain  in  Weight  of  Drying  Oils  at  Room  Temperature 

(Friend). 

in  weight  due  to  escaping  vapours  and  gases.  Any  factor 
assisting  or  retarding  the  removal  of  the  oxidation  products 
will  proportionally  lower  or  raise  the  observed  increase  in 


DRYING  OILS 


43 


the  weight  of  the  oil ;  this  accounts  for  the  varying  figures 
given,  13-25-6  per  cent.  (Ingle,  /.  5.  C.  /.,  1913,  j2,  639,  and 
Redman,  Weith,  and  Brock,  /.  Ind.  Eng.  Ghent.,  1913, 5,  630.) 


I.  Cotton5Seed[Oi1.       2.  WalnutlOil.       3,  Linseed  Oil. 
Fig.  4. — Rate  of  Percentage  Gain  in  Weight  of  Oils  at  100°  C. 


In  Figs.  I  and  2  the  variation  of  the  percentage  of 
oxygen  absorbed  and  the  increase  in  weight  of  linseed 


44   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


oil  with  the  time  of  exposure  are  shown.  In  Figs.  2  and  5 
a  comparison  is  given  between  drying  oils  and  semi-drying 
oils,  as  to  the  weight  of  oxygen  absorbed  with  the  time. 

In  Figs.  4  and  5  the  effect  of  temperature  on  the  rate  of 
absorption  by  three  oils,  of  which  cotton  seed  oil  is  a  typical 
semi-drying  oil.  From  an  examination  of  the  curves  there 
is  evidence  of  an  induction  period  during  which  the  increase 
in  weight  is  slow.  It  is  assumed  that  small  quantities  of 
peroxides  are  being  produced  which,  on  their  formation,  act 
catalytically  as  oxygen  carriers.  The  addition  of  a  metallic 
8 


Fig.  5.— Percentage  Gain  in  Weight  of  Drying  Oils  at  100°  C.  (Friend). 

drier  reduces  this  induction  period.  Fokin  (/.  Russ.  Phys. 
Chem,  Soc,  1907,  jp,  607,  and  1908,  40,  276)  states  that  the 
rate  of  setting  of  linseed  oil  films  follows  Spring's  rule,  in 
that  it  is  doubled  for  every  10  degrees  rise  in  temperature  ; 
moisture  retards  the  time  of  setting.  It  is  evident  that  the 
change  cannot  be  expressed  by  a  simple  velocity  equation 
because  of  the  loss  of  volatile  products  and  the  possibilities 
of  polymerization  of  the  oxidizing  and  oxidized  oil.  Poly- 
merization would  tend  to  reduce  the  amount  of  the  volatile 
products. 

The  influence  of  moisture  is  shown  in  Fig.  6.  / 


DRYING  OILS 


45 


The  curves  in  Fig.  6.  (A.  de  Waele,  /.  5.  C.  /.,  1920,  jp, 
48T)  represent  graphically  the  results  obtained  by  plotting 
the  variations  in  weight  shown  by  exposing  variously 


?? 


O     0^00      t^vo     lOThrow      h      O      ONOO      t^vo      m-^CDN      H  O 


Increase  in  weight  % 

ro  Ti-  uno  t^co  o>  O  m  n      tj-  ii-)>o  t»» 

P.W.V.  (miTlimetres^Hgr)  " 

-<*-M  OOOVO  "^(N  OcovO  M-N  O 

Relative  Humidity 

treated  linseed  oils  in  air,  the  atmospheric  conditions  being 
simtdtaneously  plotted  as  detailed  in  the  graph.  It  will 
be  noticed  that  the  two  series  of  observations,  viz.  in  hght 
and  in  darkness,  although  agreeing  in  the  direction  of  the 


46  RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


path  of  the  curve  followed  by  the  individual  oils  examined, 
differ  very  materially,  a  fairly  close  connection  existing 
between  the  inverse  of  the  pressure  of  the  water  vapour 
and  the  light  conditions,  whilst  the  darkness  conditions 
follow  more  closely  the  relative  humidity  readings.  The 
coincidence  of  the  paths  followed  by  the  different  oils  occurs 
after  the  completion  of  the  first  induction  period.  The 
relative  stability  of  polymerized  oil  is  shown  by  the  steady 
upward  tendency  and  closeness  of  the  ordinates  at  the 
33rd  and  133rd  day  periods,  and  further  supports  the  view 
as  to  its  constitution  outlined  on  p.  37.  The  conclusions 
drawn  by  the  author  from  the  above  curves  are  that  the 
periodic  variations  in  weight  are  caused  by  the  two  factors 
composing  the  atmospheric  conditions :  (i)  The  decom- 
position of  the  primarily  formed  peroxides  is  initiated  by 
the  presence  of  water ;  and  (2)  equilibrium  conditions, 
i.e,  decomposition  versus  back-pressure,  are  attained  by 
the  interpretation  of  the  latter  as  pressure  of  water  vapour. 

Gardner  {Circular  No,  70,  Paint  Manufacturers  Assoc,  of 
the  US.),  discussing  variation  in  weight  curves  showing 
two  alternate  increases  and  decreases,  holds  the  view  that 
the  first  decrease  is  represented  by  decomposition  of  the 
oxidized  product  into  volatile  products  and  solid  oxidized 
residue,  followed  by  splitting  up  of  the  latter  into  fatty 
acid  and  the  glyceryl  radicle,  the  glyceryl  then  taking  up 
water  to  form  glycerol  and  thus  representing  the  second 
increase.  The  second  decrease  is  then  held  to  be  accounted 
for  by  decomposition  of  the  glycerol  into  volatile  decom- 
position products. 

The  effect  of  stoving  a  linseed  oil  film  at  lOo""  C.  is  to 
increase  the  water-resisting  power  by  increasing  the  thickness 
of  the  layer  of  linoxyn.  Friend  has  found  that  a  film  of 
linseed  oil  dried  at  15°  C.  absorbed  five  times  as  much 
moisture  as  a  similar  film  stoved  at  100°  C.  (/.  Iron  and  Steel 
Institute,  1911,  iii.  54) .  He  states  that  the  painting  of  wrought 
iron  while  it  is  hot  gives  a  much  more  effective  protection. 

Genthe  (Z.  angew.  Chemie,  1906,  ip,  2087),  Fig.  7,  showed 
that  the  rate  of  oxidation  of  linseed  oil  exposed  to  ultra-violet 


DRYING  OILS 


47 


light  was  greatly  accelerated.  In  the  dark  the  maximum 
absorption  was  reached  in  50  days,  whereas  when  exposed 
to  light  from  a  mercury  lamp  the  maximum  was  attained 
in  25  hours.  Under  these  conditions  linseed  oil  was  found 
to  absorb  34  per  cent,  of  its  weight  from  the  atmosphere. 

Changes  in  weight  are  accompanied  by  changes  in 
volume.  Friend  {Trans.  Chem.  Soc,  1917,  iii,  162)  found 
that  with  increase  in  weight  (17 '9-18 '5  per  cent.,  i,e,  solid 
linoxyn),  there  was  an  increase  in  density  with  an  increase 
in  volume  up  to  3  per  cent.  ;  subsequently  a  contraction 
ensues  on  long  exposure  which  is  a  simple  explanation  of  the 


cracking  of  paints  and  the  shrivelling  of  varnishes.  Wolff 
{Farb.  Zeit.,  1919,  24,  1119)  has  found  that  where  varnishes 
yield  glossy  films  in  light  of  short  wave-lengths,  shrivelled 
films  are  produced  in  light  of  long  wave-length.  He 
maintains  {loc.  cit.,  1389)  that  in  varnishes  exposed  to  light 
of  short  wave-length  oxidation  and  polymerization  proceed 
at  nearly  equal  rates,  whereas  in  light  of  long  wave-length 
polymerization  is  retarded  more  than  oxidation,  so  that 
inequality  in  volume  of  inner  and  outer  layers  is  produced. 

Gardner  (J.  Ind.  Eng.  Chem.,  1914,  6,  91)  has  shown  that 
linseed  oil,  when  mixed  with  chemically  inert  powders  such 
as  barytes  and  silica,  exhibited  a  smaller  gain  in  weight  on 


Fig.  7.—^ 


48   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


setting,  the  powders  apparently  assisting  in  the  decomposition 
of  the  peroxides  with  the  production  of  volatile  material. 

The  absorption  of  oxygen  is  accompanied  by  an  evolution 
of  heat,  so  that  precautions  must  be  taken  to  avoid  sponta- 
neous combustion  of  any  fabric  impregnated  with  a  drying 
oil  (Mackey  and  Ingle,  J.  5.  C.  1917,  j6,  319).  Similarly 
the  absorption  of  bromine  by  linseed  oil  is  exothermic,  and 
Harden  (/.  Ind.  Eng.  Chem.,  1916,  121)  states  that 
206  calories  per  gram  of  linseed  oil  are  evolved,  compared 
with  1007  calories  in  the  case  of  olive  oil. 

After  the  general  statement  of  the  properties  of  linseed 
oil  as  a  drying  oil,  it  will  be  advisable  to  describe  briefly  the 
sources  of  the  seed,  and  the  mode  of  extraction  of  the  oil. 

On  the  importation  of  linseed  depends  the  supply  of 
paint  in  the  building  and  decorating  trades,  and  for  the 
manufacture  of  linoleum,  although  abroad  linseed  oil  is  used 
to  a  very  limited  degree  as  a  burning  oil ;  during  the  war 
it  was  an  important  source  of  glycerine. 


United  Kingdom 
United  States 
Germany  . . 

France 
Belgium 


Imports  for  Consumption  of  Oil  Seeds  in  1913 

Linseed. 
654,812  tons 

132,357 

560,323  metric  tons 


237,406 
259,105 


Soya  beans. 
76,452  tons 

Not  separately  distinguished 
125,750  metric  tons 

(including  castor  oil) 
45 

Not  separately  distinguished 


1 91 3  was  considered  to  be  an  abnormally  high  year  for  linseed. 

On  conversion  of  the  seed  into  oil  values  and  deducting 
the  figures  for  re-export,  so  far  as  the  United  Kingdom  is 
concerned,  Mr.  Pearson,  chairman  of  the  Oilseed  Crushers 
Association,  has  compiled  the  following  table  for  1912-14, 
1915,  and  1916. 


Oil  imports  or  equivalent 
of  seed  imports. 

Oil  exports. 

Oil  retained  for  U.K. 

1912-14. 

1915. 

1916. 

1912-14. 

1915. 

1916. 

1912-14. 

1915. 

1916, 

Linseed 

oil 
Cotton 

seed  oil 
Fish  oil 

Tons. 
158,124 

127,510 

63,811 

Tons. 
123,554 

117,911 
91,872 

Tons. 

i5i»7io 
60,212 
109,061 

Tons. 
275,562 

26,360 
8,229 

Tons. 
55,927 

36,082 

4»570 

Tons. 
24,761 

3»397 

1,146 

Tons. 
130,562 

101,150 

Tons. 
67,627 

81,829 
87,302 

Tons, 
126,949 

56,815 

107,915 

DRYING  OILS 


49 


During  the  war  the  demand  for  oil  seeds  was  increased 
by  three  main  causes  :  (i)  the  demand  for  glycerine  for 
explosives,  which  involved  splitting  a  quantity  of  oil  far 
exceeding  the  normal  requirements  of  the  soap  trade. 
(2)  The  demand  for  edible  oils  for  margarine.  (3)  Main- 
tenance of  the  supplies  of  feeding  cake  for  cattle. 

Fortunately  the  British  Empire  possessed  the  very 
substantial  advantage  that  the  supply  of  oil  seeds  generally 
was  amply  sufficient  for  the  requirements,  and  left  a  margin 
for  export,  as  shown  in  the  table  given  above.  By  the 
system  of  prohibiting  exportation  from  countries  of  origin 
to  destinations  other  than  the  United  Kingdom  or  the 
Allies,  except  under  special  licence,  it  was  possible  to  secure 
to  this  country  any  quantities  required  for  which  shipping 
was  available. 

The  largest  exporting  countries  of  linseed  are  the 
Argentine,  Russia,  India,  and  Canada.  The  general  oil- 
seed production  of  India  is  already  very  great,  and  is  capable 
of  still  further  increasing  Oils  and  Fats  in  the  British 
Empire,"  Sir  A.  D.  Steel  Maitland,  1917).  The  Canadian 
crop  is  absorbed  by  the  United  States.  The  Argentine 
exports  to  Europe  reached  in  1913,  1,100,000  tons,  or  on  the 
average  800,000  tons  annually.  The  exportable  surplus 
from  India  varied  from  250,000  tons  to  400,000  tons  in 

1913. 

Small  quantities  have  been  grown  in  England  with  very 
satisfactory  results  (Eyre  and  Morrell,  British  Flax  and  Hemp 
Growers'  Society  Publications,  1919,  and  /.  Board  of  Agri- 
culture, 1919).  China  is  becoming  a  linseed  oil  seed  exporting 
country.  Soya  bean  from  China,  Manchuria,  and  Japan 
have  become  less  popular,  and  have  fallen  from  400,000  tons 
in  1 910  to  a  much  lower  figure. 

The  flax  or  linseed  plant  (linum  usitatissimum)  is  grown  in 
some  countries  for  fibre,  and  the  highest  germinating  seeds 
are  preserved  for  sowing.  The  true  flax-bearing  plant  has 
a  straight,  unbranched  stem,  40  inches  high,  consisting  of  a 
core,  an  outer  covering,  and  an  intermediate  layer  of  bast 
tissue.    It  is  the  bast  tissue  which  gives  the  flax  fibre, 

s.  4 


50   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


after  separation  by  retting  and  scutching,  to  be  spun  for 
linen.  The  seed  variety  is  branching  and  shorter  in  the 
stem,  and  its  fibre  is  of  relatively  small  importance.  The 
yield  of  seed  per  acre  from  the  flax  plant  is  6-10  cwt.,  while 
the  seed  variety  gives  10-20  cwt.  per  acre.  The  seed  from 
the  branching  variety  is  larger  and  yields  30-40  per  cent,  of 
oil.  To  extract  the  oil  from  the  seed  hot  or  cold  pressing 
may  be  employed,  the  former  being  the  most  common,  while 
the  latter  is  only  resorted  to  when  a  pale  edible  oil  is  required, 
as  in  Russia,  German}^  and  India. 

Extraction  of  Linseed  Oil. — The  extraction  of  oils 
from  their  seeds  is  essentially  an  engineering  problem.  The 
broad,  general  methods  are  two,  viz.  employment  of  pressure 
and  extraction  by  solvents.  As  the  seeds  are  small  they 
are  crushed  in  hydraulic  presses  and  the  oil  forced  out  of 
the  seed.  The  seed  or  meal  is  heated  during  the  process  of 
hot  crushing,  and  the  oil  produced  is  known  as  hot-pressed 
or  hot-drawn  oil.  Such  oil  is  discoloured  by  its  having 
dissolved,  during  the  expression,  an  excessive  amount  of 
colouring  matter.  The  cold-drawing  process  leaves  usually 
a  considerable  amount  of  oil  in  the  cake,  but  it  is  freer  from 
impurities  such  as  mucilage,  and  is  of  a  better  colour.  By 
either  hot  or  cold  process  not  more  than  90-95  per  cent,  of 
the  total  oil  is  removed  from  the  press-cake.  The  second 
process  extracts  all  the  oil  from  the  seed  by  the  use  of 
solvents,  benzol,  carbon  disulphide,  or  carbon  tetrachloride. 
The  process  in  outline  consists  in  allowing  the  solvent  to 
percolate  through  the  seed  or  meal  in  a  closed  vessel,  drawing 
off  the  solution  and  distilling  off  the  solvent. 

Anglo- American  Crushing  System. — I^inseed,  rape 
seed,  and  similar  small  seeds  do  not  entail  preparator}^ 
machines  to  reduce  them  to  a  form  suitable  for  treatment  in 
the  subsequent  oil  recovery  process.  Such  machines  are 
specialized  and  are  designed  to  deal  with  one  particular 
class  of  seed  or  nut,  e.g.  for  reducing  copra  and  converting 
it  to  meal  (Fig.  8). 

The  oil  seeds  (linseed,  rape,  etc.)  are  first  passed  through 
a  magnetic  separator  to  remove  pieces  of  metal  which 


DRYING  OILS 


51 


have  been  added  to  bring  up  the  weight  of  the  seed,  and 
then  pass  by  an  endless  band  conve3^or  to  the  crushing 
mills. 

Seed  Crushing. — This  operation  is  performed  by  a  crash- 
ing mill  which  consists  of  5  rolls,  16  inches  in  diameter  and  42 
inches  long,  stacked  vertically,  and  are  quite  plain  on  the  sur- 


FiG.  8. — Anglo-American  Rolls.  (Rose,  Down  and  Thompson,  Ltd.,  Hull.) 

face.  As  usual  they  are  ground  very  true  and  are  forced  on 
to  their  shafts  by  hydraulic  pressure,  and  thereafter  ke3^ed 
at  both  ends.  The  lowest  roll  is  driven  at  both  ends  and  is 
provided  with  two  additional  pulleys,  from  which  belts  are 
taken  to  similar  sized  pulleys  at  each  end  of  the  third  and 
fifth  rolls.  The  bearings  for  all  the  rolls  except  the  lowest 
are  free  to  slide  vertical^  in  their  housings,  so  that  the 
pressure  exerted  on  the  seed  increases  with  each  step  in  its 


52   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


descent.  The  capacity  of  the  rolls  is  about  15  cwt.  per 
hour. 

Heating  and  Moulding.— The  heating  of  the  crushed 
seed  or  meal  facilitates  the  expression  of  the  oil,  helping  to 
rupture  the  cells  in  which  the  oil  is  contained  ;  moreover, 
the  viscosity  of  the  oil  is  reduced  so  that  on  pressing  it  flows 


Fig.  9. — Kettle  and  Moulding  Machine  (Robert  Middleton). 
[Rose,  Down  and  Thompson,  Ltd.,  Hull.] 
("The  Production  and  Treatment  of  Vegetable  Oils,"  T.  W.  Chalmers.) 


away  more  freely.  Heating  also  coagtdates  the  albuminous 
matter  which  is  retained  in  the  press  cake.  Frequently  a 
little  steam  is  admitted  direct  into  the  kettle,  not  so  much 
to  heat  the  meal,  but  to  improve  its  condition  and  to 
facilitate  the  flow  of  the  oil.  The  time  and  temperature  of 
heating  may  be  20  minutes  and  170°  F.  respectivety. 


DRYING  OILS 


53 


The  meal  is  withdrawn  from  the  kettle  in  suitable  amounts 
and  is  immediately  rough-moulded  in  a  machine  to  the 
form  of  slabs  suitable  for  the  press  in  use,  and  as  quickly  as 
they  are  made  the  slabs  are  transferred  to  the  press. 

Pressing. — In  the  Anglo-American  press  the  interspace 
between  the  ram  and  the  top  plate  is  provided  with  a  number 
of  iron  plates  (lo- 
20),  usually  sus- 
pended evenly  one 
over  the  other.  Be- 
tween the  plates  are 
placed  cakes  of  uni- 
form size  as  they 
come  from  the  mould- 
ing machine.  On  the 
application  of  pres- 
sure the  plates  are 
forced  together,  thus 
squeezing  the  oil  from 
the  meal  and  causing 
its  exudation  through 
the  cloths  surround- 
ing each  cake. 

On  the  release  of 
the  pressure  the 
plates  fall  again  into 
their  correct  position. 
The  presses  are  de- 
signed for  a  pressure 
of  two  tons  per  square 
inch.     The  columns 


Fig.  10. — Type  of  Anglo-American  Press. 

Plates  supported  by  Links. 
(Rose,  Down  and  Thompson,  Ltd.,  Hull.) 


are  of  mild  steel  and  the  bottom  has  a  large  receptacle  for 
the  expressed  oil,  from  which  it  is  conducted  to  underground 
tanks.  The  plates  may  be  supported  by  racks,  or  by  links, 
as  in  the  figures. 

The  press  is  undoubtedly  efficient.  It  is  simple,  and 
running  in  conjunction  with  the  modern  meal-moulding 
machine,  it  is  easily  and  quickly  loaded.    It  is  rapidly 


54   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


unloaded,  but  the  stripping  of  the  press  bagging  from  the 
cakes  may  involve  considerable  labour,  so  that  special 
machinery  may  have  to  be  installed  for  mechanical  stripping. 

The  presses  are  best  adapted  for  dealing  with  seeds 
containing  a  moderate  amount  of  oil.  The  cake  meal  is 
pressed  only  on  two  sides  and  not  round  the  edges.  The 


plates  are  often  cor- 
rugated so  as  to  pre- 
vent, as  far  as  pos- 
sible, the  meal  from 
spreading,  which  is 
occasionally  success- 
ful but  not  always  so 
with  seeds  of  high  oil 
content.  For  seeds 
containing  a  high  per- 
centage oil  a  box- 
cage  type  of  press  is 
preferred  (Chalmers, 
loc.  cit).  The  press 
cakes  are  the  linseed 
cake  of  cattle  food. 

The  Extraction 
Process,— The  5-10 
per  cent,  oil  left  in 
the  press  cakes  is  not 
a  source  of  loss,  but 
the  cake  from  which 


Fig.  II.— Type  of  Anglo-American  Press.      'the  oil  has  been  en- 
Plates  supported  by  Racks,  tirely  extracted  can 
(Rose,  Down  and  Thompson,  Ltd.,  Hull.)  direct  tO 

cattle  without  requiring  dilution  with  bran  and  other 
substances,  as  is  the  case  with  the  ordinary  press  cake. 
The  oil  extraction  solvents  must  be  carefully  selected. 
Messrs.  G.  Scott  and  Sons,  lytd.,  I^ondon,  make  an  oil 
extraction  plant  in  which  benzol  is  used  as  solvent.  Benzol 
is  stated  to  be  one  of  the  most  difficult  solvents  to  remove 
from  the  oil,  but  it  is  the  safest  and  the  results  are  considered 


DRYING  OILS 


55 


to  be  satisfactory.  The  extraction  is  performed  cold.  In 
the  continuous  still  the  descending  oil  and  solvent  are  met 
by  an  ascending  current  of  steam,  so  that  the  benzol  is 
carried  away  with  the  steam.  The  working  charges  are 
very  low.  The  coal  required  per  ton  of  raw  material  may 
amount  to  two  or  three  cwt.  and  the  loss  of  solvent  may  be 
returned  at  if  galls,  per  ton  of  seed  treated.  The  statement 
by  Mastbaum  (Z.  angew,  Ghent.,  1916,  p,  719),  that  the 
extracted  oil  dries  slower  than  the  pressed  oil  must  be 
doubted.  In  the  pressed  oil  it  was  suggested  that  the  more 
fluid  glycerides,  i.e,  the  unsaturated  glycerides,  would  be 
extracted  first,  but  as  linseed  oils  contain  mixed  glycerides 
such  a  differentiation  is  not  possible  by  a  pressing  extrac- 
tion process.  For  fuller  details  respecting  the  properties 
of  linseed  oil  and  other  drying  oils  reference  may  be  made 
to  lyCwkowitsch,  Oils,  Fats,  and  Waxes,''  5th  edition,  and 
to  Fryer  and  Weston,    Oils,  Fats,  and  Waxes,"  1917. 

A  short  description  of  the  properties  of  several  paint  and 
varnish  oils  must  be  given,  viz.  Perilla,  Soya,  Poppy-seed, 
and  China  wood  oil  (Tung  oil). 

Perilla  Oil  is  expressed  from  the  seeds  of  Perilla  Ocimoides 
{N ankinensis) y  an  annual  plant  growing  in  China,  Japan,  and 
the  East  Indies.  In  Japan  it  is  employed  as  an  adulterant 
of  lacquer.  Very  little  is  exported  to  the  United  Kingdom. 
In  Manchuria  it  is  stated  to  be  used  for  edible  purposes.  In 
spite  of  its  high  iodine  value  (206)  and  high  insoluble  bromide 
value  of  its  fatty  acids,  its  drying  power  is  stated  to  be 
inferior  to  that  of  linseed  oil.  The  statement  that  it  forms 
drops  "  when  spread  on  a  surface  is  not  generally  accepted 
by  users  of  the  oil.  H.  A.  Gardner  Paint  Researches," 
1907)  reports  favourably  on  the  use  of  perilla  oil  in  paints 
and  linoleum. 

Soya  Bean  Oil. — The  soya  bean  [Glycine  hispida)  is 
indigeneous  to  China,  Manchuria,  and  Japan.  It  was 
practically  unknown  in  this  country  before  1908,  or  until 
the  Russo-Japanese  war.  Since  that  time  the  use  of  oil  and 
cake  has  spread  phenomenally. 

Of  late  the  demand  has  dropped,  although  the  oil  cake  as 


56    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


a  cattle  food  is  a  rival  to  linseed  and  cotton  cake.  The  oil 
belongs  to  the  less  active  drying  oils,  although  with  driers 
it  gives  a  fair  drying  oil  (Gardner,  Circular  No.  69,  Paint 
Manufacturers  Assoc.  of  the  U.S.A.,  Aug.,  1919).  The  low 
iodine  value  (130)  with  an  insoluble  bromide  value,  7*8, 
show  that  its  oxygen  absorbing  power  is  small.  The  oil  is 
in  demand  in  large  quantities  for  edible  purposes,  but  its 
use  in  paints  is  limited.  The  beans  contain  18  per  cent, 
oil,  from  which  10-13  P^^  cent,  can  be  extracted  by  either 
native  methods,  or  by  the  Anglo-American  process.  The 
emulsification  of  the  oil  with  gluten  or  casein-like  bodies  in 
the  beans  is  the  basis  of  some  patents  for  artificial  milk 
(Melhuish,  B.P.  24572  (1913),  and  Eng.  Pat.,  9626  (1915), 
also  ''B.  A.  Reports  on  Colloid  Chem.  and  its  Industrial 
x\pplications,''  1918,  106).  The  emulsifying  power  of  the 
oil  seems  superior  to  that  of  other  drying  oils. 

Pararubber  Seed  Oil. — This  oil,  obtained  from  Para 
rubber  seed  kernels,  has  been  investigated  recently  at  the 
Imperial  Institute  {Bulletin  of  the  Imperial  Institute,  1913, 
vol.  xi.  551).  It  dries  less  quickly  than  linseed  oil,  but  it 
would  be  a  valuable  substitute  when  linseed  oil  is  high  in 
price.  For  linoleum  manufacture  it  is  considered  to  be 
unsuitable.  The  usual  constants  of  the  oil  are  :  sp.  gr.,  15°, 
0*925  ;  acid  value,  i6'8  ;  saponification  value,  192*1  ;  iodine 
value,  131-138.    See  also  pp.  29,  30. 

Poppy-seed  Oil. — The  seeds  of  the  opium  poppy  {Papaver 
somniferum)  are  grown  in  India,  Russia,  France,  and  Asia 
Minor,  and  contain  40-50  per  cent,  of  an  oil  which  when 
cold-drawn  is  almost  colourless  (white  poppy-seed  oil),  and 
possesses  a  pleasant  taste.  The  cake  is  rich  in  nitrogen, 
and  is  highly  prized  as  a  cattle  food.  The  oil  is  a  fair  drying 
oil  and  is  used  more  in  artists'  colours  than  in  ordinary 
paints.  The  seeds  are  generally  expressed  twice,  the  second 
pressing  being  carried  out  hot  and  yielding  an  inferior  oil 
which  is  extensively  used  for  soft  soaps.  The  colourless  oil 
is  also  used  for  adulterating  olive  oil. 

In  describing  the  oil  obtained  from  any  seed  or  nut  it 
must  be  remembered  that  the  utilization  of  the  press  cake 


DRYING  OILS 


57 


is  a  prime  factor  in  the  successftil  application  of  an  oil.  If 
the  cake  is  useless  the  development  of  the  oil  is  retarded. 
It  is  for  this  reason  that  attention  has  been  drawn  to  the 
usefulness  of  the  press  cake  in  the  description  of  the  several 
oils.  Poppy-seed  oil  is  said  to  contain  65  per  cent,  linolic 
acid,  5  per  cent,  linolenic  acid,  and  30  per  cent,  oleic  acid 
(Hazura).    The  iodine  value  is  131-157. 

China  Wood  Oil  (Tung  Oil). — The  seeds  of  Aleurites 
cordata  are  contained  in  a  pod  or  nut  about  the  size  of  a 
small  orange  containing  usually  about  three  seeds.  These 
seeds  can  easily  be  distinguished  from  those  of  linseed  by 
their  size. 

The  seeds  are  treated  by  crude  native  methods,  and  the 
yield  is  40  per  cent,  from  an  oil  content  of  53  per  cent.  The 
cold-pressed  oil  is  pale  and  is  generally  exported.  The  hot- 
pressed  oil  is  dark  in  colour.  The  oil  cake  is  poisonous 
and  can  be  used  only  as  a  fertilizer.  In  China  it  is  the  custom 
to  impregnate  the  wood  of  boats  with  the  oil,  hence  the  name 
China  wood  oil  (Seeligman). 

Recently  the  cultivation  of  the  tree  has  been  undertaken 
in  the  Southern  States  of  North  America.  In  1914,  40,000 
trees  were  in  cultivation  and  the  results  were  satisfactory. 
No  doubt  the  trouble  respecting  the  unsuitability  of  the 
cake  as  a  cattle  food  will  be  overcome,  and  the  application 
of  modern  methods  of  crushing  and  refining  on  the  spot 
will  enhance  the  importance  of  cultivation  of  Aleurites. 
In  1906  nearly  29,000  tons  were  exported  from  Hankow. 
The  literature  on  tung  oil  is  very  extensive  Index  to 
Patents,  Technology  and  BibHography  of  China  Wood  Oil,'' 
A.  H.  Stevens  and  J.  W.  Armitage).  Tung  oil  has  marked 
peculiarities  differing  from  linseed  oil  in  the  following  re- 
vSpects  : — It  dries  in  about  two-thirds  the  time  of  linseed  oil, 
giving  a  film  which  is  white,  dull,  opaque,  and  crinkled. 
These  effects  are  much  reduced  in  the  presence  of  driers,'' 
but  they  are  especially  marked  if  drying  be  retarded  in  a 
gas-laden  or  foul  atmosphere.  The  surface  puckers  or 
webs,  and  becomes  matt  with  a  finely  radiating  crystalline 
appearance. 


58    RUBBER,  RESINS,  TAINTS,  AND  VARNISHES 


Often  the  film  may  become  uniformly  opaque  with  a 
ground  glass  appearance  which,  when  examined  under  the 
microscope,  is  seen  to  be  similar  to  the  above.  If  the  drying 
is  carried  out  at  temperatures  above  80°  C.  the  film  is  trans- 
parent and  smooth.  The  change  may  be  due  to  formation 
of  a  solid  isomer ;  such  an  isomer  is  produced  when  wood 
oil  is  exposed  to  light.  The  crystalline  isomer  is  readily 
oxidized  to  a  white  solid  peroxide  (Morrell,  Traits,  Chem, 
Soc,  1912,  loi,  2082).  Another  peculiarity  is  the  rapid 
gelatinization  of  the  oil.  When  heated  to  288°  C.  for 
9  minutes,  it  sets  to  a  hard  transparent  jelly.  This  property 
can  be  used  for  detection  of  impurities  in  the  oil,  because  the 
addition  of  12  per  cent,  of  another  oil,  e.g.  soya  bean  oil, 
will  retard  the  gelatinization  at  that  temperature.  The 
change  is  due  to  polymerization  which  differs  greatly  in 
rate  from  that  of  linseed  oil. 

Schumann  (/.  Ind.  Eng,  Chem,,  1916,  8,  5)  has  put 
forward  tne  view  that  a  dipolymeride  is  first  formed  which 
afterwards  gels.  He  discusses  the  mode  of  preventing  the 
coagulation,  which  is  a  most  undesirable  property  although 
the  rapid  formation  of  the  dipolymeride  is  of  great  importance 
and  its  proper  control  has  been  the  subject  of  much  investi- 
gation. Careful  research  is  gradually  overcoming  the 
difiiculties  due  to  the  rapid  gelatinization.  The  durability 
of  the  treated  tung  oil  is  generally  superior  to  that  of 
linseed  oil,  and  when  the  defects  of  webbing  and  coagulation 
are  overcome  by  the  manufacturer  he  can  produce  a  much 
improved  protective  coating,  but  skill  and  judgment  are 
necessary  in  the  treatment  of  the  oil.  Owing  to  the  rapidity 
of  coagulation  it  is  very  difficult  under  ordinary  conditions 
to  incorporate  copal  resins  with  raw  wood  oil  in  a  varnish 
mixing. 

It  may  be  mentioned  that  the  isomer  of  wood  oil  is  the 
only  crystalline  drying  oil  which  has  been  isolated  in  a  pure 
condition  (Morrell,  loc.  cit.). 

The  following  table  shows  a  comparison  of  the  constant 
of  linseed  and  China  wood  oils  : — 


DRYING  OILS 


59 


Linseed  oil. 

0'933 
1-4831 


China  wood  oil. 


Sp.  gr.  (15°  C.) 


0-  9405 

1-  5174  (12-5' 
3 '3 


ft     ,  .        ,  ,        .  ,        ,  , 

D.  acidity  .  . 
Saponification  value 
Iodine  value 

Per  cent,  increase  in  weight 


n 


,°  C.) 


0-4 

197 
185 


igz 
168 


on  exposure  to  air 


17-6  (SJ  days) 


13-5  (8 J  days) 


The  importance  of  its  metallic  salts  will  be  referred  to 
under  the  theories  of  drying.  Boiled  with  caustic  alkalies, 
it  yields  soaps  which  are  granular  in  comparison  with 
ordinary  soaps,  since  in  the  pure  state  the  alkali  salts  of 
elaeostearic  acid  (C18,  H32,  O2),  of  which  tung  oil  is  the 
glyceride,  are  crystalline  substances  which  are  sparingly 
soluble  in  water.  Owing  to  the  small  amount  of  impurity 
China  wood  oil  lends  itself  readily  to  favourable  investiga- 
tion, the  only  foreign  component  (with  the  exception  of  the 
dissolved  solid  isomer)  known  at  present  is  oleic  acid  to 
the  amount  of  10  per  cent.  (Fahrion  states  that  2-3  per 
cent,  solid  acids  are  present.)  It  gives  no  other  insoluble 
bromide  either  as  glyceride,  or  when  transformed  into  the 
free  acid.  A  solid  tetrabromide  of  the  same  melting  point 
has  been  obtained  from  the  two  forms  of  glycerides. 

Its  peculiar  drying  properties  must  be  connected  with 
the  orientation  of  the  double  linkages  in  the  molecule,  since 
it  is  isomeric  with  the  glyceride  of  linolic  acid  (a  much 
slower  drying  oil)  which  is  present  in  poppy-seed  oil  to  the 
extent  of  about  65  per  cent. 

lyinolic  acid, 

CH3(CH2)4CH  :  CH.CH2.CH  :  CH(CH2)7COOH 

(Goldsobel,  J.  Russ,  Phys.  Chem,  Soc,  1906,  jS,  182). 
Elaeostearic  acid, 

CH3(CH2)3CH  :  CH[CH2]2.CH  :  CH.[CH2]7C02H 

(Majima,  Ber.,  1909,  42,  674  ;  1912,  ^5,  2730). 

If  proper  account  be  taken  of  the  peculiarities  of  tung 
oil,  resin  mixings  containing  it  are  superior  in  durability  to 
those  prepared  with  linseed  oil,  but  the  conditions  require 
careful  attention  and  neglect  of  them  results  in  highly 
undesirable  products. 


6o    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Other  oils  in  use,  in  small  quantities,  for  paints  and 
varnishes  are  : — 

Walnut  Oil. — A  very  pale  oil  used  by  artists,  because 
paints  made  with  it  have  a  less  tendency  to  crack  than 
those  containing  linseed  oil. 

Japanese  Wood  Oil. — Obtained  from  Paulownia  impe- 
rialis;  it  is  very  similar  to  China  wood  oil,  but  gelatinizes 
less  readily. 

Candlenut  Oil,  Niger  Oil,  Sunflower  Oil,  and  Hemp- 
seed  Oil  possess  drying  properties  whereby  they  may  be 
used  in  paints  and  varnishes,  but  their  drying  properties  are 
inferior  to  those  of  linseed  oil.  Provided  the  price  of  linseed 
oil  is  not  much  higher  than  that  of  the  five  oils  mentioned, 
the  demand  for  them  is  very  limited  and  the  supply  is  much 
smaller.  They  are  used  chiefly  for  soap  making,  or  for 
edible  purposes. 

Among  the  animal  drying  oils  menhaden,  or  fish  oil,  can 
be  used  as  a  drying  oil.  The  production  from  Alosa 
menhaden  (a  fish  resembling  a  herring)  amounts  to  nearly 
half  a  million  tons  per  annum,  from  the  Atlantic  coastline 
of  North  America.  It  is  cheaper  than  the  vegetable  oils, 
and  is  used  for  adulterating  paints  or  for  leather  cloth  or 
oil  tablecloths,  in  the  United  States.  It  is  not  popular  in 
this  country  for  paints  and  varnishes  because  of  its  odour 
and  general  inferiority  to  linseed  oil.  Toch  (/.  Ind,  Eng. 
Chem.,  1911,  J,  627)  recommends  a  mixture  of  75  per  cent, 
fish  oil  and  25  per  cent,  linseed  oil  for  outside  paints,  but 
generally  its  durability  is  inferior  to  linseed  oil.  It  owes  its 
drying  powers  essentially  to  the  presence  of  clupanodonic 
acid  (which gives  an  insoluble  octobromide),  but  its  chemical 
composition  is  not  fully  known.  The  oil  content  of  Alosa 
menhaden  is  about  15  per  cent. 

Resin  oil  and  pine  oil  will  be  referred  to  in  the  chapter  on 
Resins. 

Boiled,  Blown,  and  Stand  Oils  (Lithographic  Var- 
nishes).— lyinseed  oil  when  heated  to  22o''-28o'' C.  for  several 
hours  in  contact  with  the  air  is  said  to  have  its  power  of 
absorbing  oxygen  increased  (Hurst  and  Heat  on,  Painters' 


DRYING  OILS 


6i 


Colours,  Oils,  and  Varnishes  It  is  customary  to  add  small 
quantities  of  metallic  driers  whereby  the  resulting  boiled 
oil is  essentially  an  oil  containing  a  metallic  catalyst. 

The  oil  may  be  heated  over  a  fire  in  large  boilers  holding 
100-600  gallons :  the  boilers  are  made  of  wrought-iron 
plates  of  the  form  shown  in  Fig.  12,  and  the  bottom,  which 
is  made  separately,  is  riveted  to  the  sides.  The  corroding 
action  of  the  fire  is  greatest  on  the  bottom  of  the  boilers, 
which  are  thus  replaceable.  They  should  be  set  over  furnaces 
with  fireplaces  outside  the  boiling  shed.  A  suitable  cover 
should  be  fixed  on  the  pot  to  allow  the  ingress  of  air  and  the 
removal  of  the  irritating  and  acid  fumes  from  the  boiling 
oil.  The  oil  in  the  pots  (not  more  than  two-thirds  full) 
is  carefully  heated  up  to  280°  C.  Great  caution  must  be 
exercised  at  first  otherwise  est 
the  oil  may  boil  over  owing 
to  the  presence  of  water  and 
of  mucilage.  After  a  time 
the  oil  boils  quietly  and 
is  kept  at  280°  C.  for  not  less 
than  2  hours.  After  it  has 
been  boiling  for  J  hour  a  small 
quantity  of  driers  is  added  Fig.  12. — Oil-boiling  Pan. 
from  time  to  time.    The  time 

of  boiling  and  the  quantity  of  driers  varies,  but  generally  5  lbs. 
per  ton  of  oil  and  5|  hours  heating  are  adequate.  The  cold 
clear  oil  is  drawn  off  as  boiled  oil  and  the  foots  are  used  for 
putty  or  mixed  into  cheap  paints.  The  oil  darkens  in  colour 
during  the  boiling  due  to  the  presence  of  decomposition 
products  depending  on  the  temperature  of  the  operation 
and  the  nature  and  quantity  of  the  driers  used.  Manganese 
dioxide  produces  a  darker  oil  than  litharge.  lycad  and 
manganese  acetates,  or  manganese  oxalate,  produce  the  palest 
oils.  The  temperature  of  the  boiling  process  should  be  kept 
as  low  as  possible  so  as  to  prevent  excessive  darkening  of 
the  oil.  The  loss  in  weight  of  the  oil  is  small,  considerably 
under  J  per  cent,  even  after  boiling  for  14  hours.  The 
direct  heating  of  linseed  oil  over  a  fire  is  being  superseded  by 


62    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


a  steam  process.  The  oil  is  run  into  a  tank  fitted  with 
a  closed  steam  coil  and  heated  for  about  2  hours  at  a 
temperature  of  203°-2o6°  C.  This  preliminary  treatment 
probably  assists  in  coagulation  of  the  mucilage  and  reduces 
the  frothing  of  the  oil  at  the  next  stage  of  the  process. 

The  hot  oil  is  run  into  a  steam-jacketed  boiler  provided 
with  a  stirrer  and  an  air  inlet  as  in  Fig.  13.  The  oil  is 
heated  by  the  steam  jacket  and  air  is  blown  through 
it.  The  driers  are  introduced  from  time  to  time,  and  on 
completion  of  the  process  the  oil  is  run  into  settling  tanks 
(Vincent,  /.  S.  Arts,  1871).  The  oil-boiling  plant  supplied 
by  Messrs.  Rose,  Down  and  Thompson 
is  on  the  same  general  principle, 
differing  in  mechanical  details.  Dur- 
ing the  boiling  with  driers  it  is  evident 
that  the  incorporation  of  the  driers 
provides  a  drying  oil.''  In  the 
presence  of  the  air  oxidation  un- 
doubtedly occurs  with  improvement 
in  the  body  and  gloss  of  the  oil.  It 
must  be  pointed  out  that  the  peroxides 
of  the  more  active  components  of 
linseed  oil  may  cause  oxidation  and 
polymerization  of  the  less  unsaturated 
glycerides  whereby  thickening  ensues. 
The  small  quantity  of  the  driers  assist 
in  the  production  of  the  peroxides. 
The  action  of  the  driers  is  to  shorten  the  slow  initial  oxida- 
tion as  shown  in  the  accompanying  diagram. 

Boiled  oil  is  of  a  brown  colour  of  varying  depth  of  shade. 
Its  colour  is  characteristic  and  peculiar,  different  from  that 
of  linseed  oil.  The  specific  gravity  varies  between  0*933 
and  0*952,  depending  on  the  mode  of  manufacture  :  the 
drying  time  is  likewise  variable  from  5-20  hours.  It 
gives  a  hard  lustrous  surface  which  is  liable  to  crack  on 
exposure  to  air,  and  is  therefore  mixed  with  raw  oil  so  as  to 
obtain  a  more  elastic  coat.  Exposure  to  light  is  stated  by 
some  to  darken  the  oil  although  linseed  oil  is  bleached  under 


Fig.  13.  — A,  Steam 
Jacket.  B,  Centre  for 
Vertical  Shaft.  C, 
Agitators.  D,  Oil  (in- 
let). F,  Flue.  L.  Air 
into  Oil.  S,  Vertical 
Shaft.    T,  Steam 


DRYING  OILS 


63 


similar  circumstances.  The  properties  which  the  consumer 
requires  for  use  in  paint  and  for  coating  wood,  masonry,  and 
metal  (generally  iron)  are  consistency,  cleanness,  drying 
power,  and  freedom  from  added  foreign  impurities,  e.g.  rosin, 
rosin  oil,  mineral  oil,  and  other  vegetable  oils. 

It  has  been  stated  that  boiled  oil  is  essentially  a  linseed 
oil  containing  driers,  and  if  this  is  strictly  correct  boiled  oil 
might  be  produced  by  merely  incorporating  driers  with  the 
oil,  but  genuine  boiled  oil  possess  body  and  gloss  due  to 
oxidation  and  polymerization  produced  in  its  mode  of 
treatment. 

The  Absorption  of  Oxygen,  by  Raw,  Boiled,  Blown,  and  PolYxMerized 

Oils. 

(Increase  in  Weight.) 

Olive  Oil  (in  winter  time)  . .  0*67%  in  2  days  1*58%  in  5  days  3*39%  in  12  days  5'6%  in  24  days 
Baltic  Linseed  Oil  (in  winter 

time)   3-8%   „  3|  ,,     i3'6%   „  6|  „    17-62     „    8f  (maximum) 

China  Wood  OU  (in  winter 

time)    1173%  „  6|  „     13-46     „    8|  „ 

East  Indian  Linseed  Oil  (in 

summertime)      ..        ..  15-9      „  16*3%  „  6^  „     14*2%    ,,    9  days 

East    Indian    Linseed  Oil, 

thickened,    500-530  (32 

hours)  (winter  time)       ..    0*56     „  sf  2*0     „  6|  „       2*62%,,    8|   „    io'9%  in  32  days 

Polymerized  Oil— Stand  Oil 

(summer)  ..        ..        ..4*9      ,,       »»       8*9     »»       »>       9'^      >»  "    io*8%  ,,27 

Boiled  Oil  (containing  lead) 

(summer)  14-8      „  i  day    14*0     „  3I  „      12-5%  „  28  „ 

Boiled  Oil  (containing  man- 
ganese and  lead  (summer)  15*2      ,,  16  hrs.  14*6     ,,  2|         I2*8      „  23  ,, 

Double  Boiled  Oil  (summer)  16-4      „  16    „    14*2     „  2I  „ 

Linseed  Oil +  2%  lead  man- 
ganese resinate  (no  heat) 

(winter)  14-1      „    5    „    i7'5     „  8J  hrs.  (film  dry) 

Linseed  Oil,  blown  at  100- 
140°  C.    (3     years  old) 

(summer)   9%   „      days  15-1    „  2J  days  137%  in  4 1  days 

Cold  Blown  Linseed  Oil  (Cal- 
cutta) Pb  (summer)       ..    2*2%    ,,  3|        11-5%,,  5     „     167      „    8     ,,    (film  dry) 

Cold  Blown  Linseed  Oil 
with    2%    Mn  resinate 

(winter)  i2-8       „  5  hrs.    i6'7%  „  8|  hrs.  17*1      „    i  day    17*6  in  i|  days 

(film  dry) 

Blown  at  150°  C.  ditto,  with 

2%  Mn  resinate  (winter)  14-5  ,,         16-5    ,,       ,,      16*5      „       „       17*6  in  days 

Linseed  Oil  heated  to  280°  C.  (film  dry) 

and  2%  Pb.  Mn.  resinate 

added   15-3      „    „         17-4    „       „      17-4     „       „       17-4  „  5  days 

(film  dry) 

Cf.  Andes'  "  Drying  and  Boiled  Oils."  1901. 

From  the  figures  in  the  table  it  will  be  evident  that  the 
gain  in  weight  and  the  rate  of  drying  is  increased  if  air  is 
blown  into  the  oil  during  the  heating,  whereas  it  has  little 
influence  on  the  cold  oil.  The  gain  in  weight  is  dependent 
on  many  factors,  viz.  temperature  and  general  atmospheric 


64   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


conditions,  and  is  a  balance  between  the  true  gain  and  the 
loss  in  weight  due  to  the  escape  of  volatile  oxidation  products. 
The  viscosity  of  the  oil  is  increased  during  the  blowing 
process  and  the  unsaturated  acids  will  have  undergone 
extensive  oxidation  to  give  glycerides  from  which  acids  can 
be  obtained  insoluble  in  light  petroleum  (Fahrion  has  shown 
that  the  oxidized  unsaturated  acids  are  insoluble  in  low 
boiling-point  petroleum).  With  reference  to  the  durability 
of  coatings  produced  from  boiled  oils  there  would  appear  to 
be  differences  of  opinion  (Ingle  and  Woodmansey, /.  5.  C.  /., 
1918,  103).  If  in  the  oxidation  a  high  proportion  of 
peroxide  glycerides  is  left  in  the  oil  it  is  to  be  expected  that 
the  superoxidized  oil  of  Reid  (/.  Soc,  Arts,  1891,  jp,  398) 
wovild  predominate  to  the  detriment  of  the  durability,  but 
if  an  oxide  stage  be  attained  durability  would  be  expected. 
The  oxidized  oil  produced  by  blowing  with  a  high  percentage 
of  peroxides  is  decomposed  by  lengthy  exposure  with  the 
formation  of  liquid  acidic  compounds  (Ingle  and  Wood- 
mansey,  loc.  cit,).  The  conditions  of  blowing  oils  must  be 
carefully  controlled  because  rapidity  of  surface  drying  is 
not  the  only  requisite,  but  the  linoxyn  layer  must  extend 
throughout  the  mass  of  the  oil,  which  would  be  attained  on 
the  assumption  that  the  peroxide  autocatalyst  was  reduced 
to  the  oxide  stage  AOg+A^^AO+O.  (A  is  the  oil 
glyceride.) 

Such  oxides  are  most  probably  polymerized  or  they  may 
undergo  transformation  into  ketones  of  the  formula 
[X— CH2^C0-- Y],  although  there  is  as  yet  no  definite 
experimental  proof  of  such  a  change.  The  peroxides  as 
such  may  undergo  decomposition  into  resinified  aldehydic 
glycerides  and  volatile  products  or  be  transformed  into 
hydroxyketones  X-CO-CHOH-Y  (Ingle,  /.  5.  C.  1913, 
j2),  but  of  this  last  change  there  is  no  definite  experimental 
confirmation.  The  view  of  the  polymerization  of  the 
products  of  oxidation  tending  to  durability  is  supported  by 
the  fact  that  the  permanency  of  polymerized  oil  is  superior 
to  that  of  an  oxidized  in  a  paint  vehicle. 

Lithographic   Varnishes   and   Stand   Oil.— There 


DRYING  OILS 


65 


would  seem  to  be  some  confusion  as  to  the  exact  meaning  of 
the  term  stand  oil."  Hurst  and  Heaton  Painters'  Colours 
and  Varnishes  ")  state  that  when  linseed  oil  is  maintained  at  a 
high  temperature  for  some  time  in  the  presence  of  air,  but 
without  driers,  it  slowly  polymerizes,  thickening  to  a  viscous 
substance  of  the  consistency  of  honey  and  consisting  largely 
of  linoxyn.  Andes  considers  stand  oil  to  be  a  linseed  oil 
thickened  by  heat  or  by  superheated  steam  and  a  current 
of  air.  A.  C.  Wright  ("Analysis  of  Oils,''  1903)  states  that 
it  is  identical  with  lithographic  varnishes.  Extremely 
thick  stand  oil  is  called  printer's  varnish  in  English ;  as  a 
painter's  material  this  oil  finds  no  application.  From  a 
summary  of  the  authorities  it  is  advisable  to  consider 
stand  oil  as  linseed  oil  polymerized  by  heat  without  driers 
and  without  blowing  "  {Oil  and  Colour  Trades  Journal^ 
("Stand  Oil"). 
When  linseed  oil  is  heated  out  of  contact  with  air  poly- 
merization ensues  with  a  rise  in  the  molecular  weight  of 
the  oil  (Morrell,  /.  5.  C.  /.,  1915,  j6,  105).  The  following 
table  illustrates  the  course  of  polymerization  in  linseed  oil 
heated  out  of  contact  of  air  (de  Waele).  The  oil  (Baltic  oil) 
was  heated  in  an  atmosphere  of  carbon  dioxide  at  250°  C. 


Raw  oil.         12  hrs.  56  hrs.  77  hrs.  (solid). 

Sp.  gr.  i5-5«  C  0-9351        0-9423  0-9664  — 

1-4808        1-4835  1*4936  1-4790 

196*6  i75'2  119-8  — 

0-78          0-29  0-5  3-24 


^D.,  25^  C. 
Iodine  Value 
Oxidized  acids  %  . . 
Saponification  Value 
Solid  acids  *        . . 

Iodine  Value  of  solid  acids    17-5  —  —  86-4 


190-7  186-0 
5-25  7-03  —  48-8 


There  was  a  notable  increase  in  the  proportion  of  solid 
acids  "  ;  they  were,  however,  transparent  and  of  a  gummy 
consistency.  By  boiling  the  solid  product  with  amyl  alcohol, 
de  Waele  obtained  an  extremely  tough  insoluble  residue 
which  had  an  iodine  value  of  94  and  was  very  resistant  to 
the  action  of  alkalies.  This  observer  believes  it  to  be  a  closed 
chain  compound  and  proposes  for  it  the  name  of  cyclolin." 
(lycwkowitsch,  "Oils,  Fats  and  Waxes/'  5th  ed.,  V0I.J3, 
P-  125.) 

*  Fachini  and  Dorta's  acetone  method. 

s.  5 


66   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Morrell  has  found  {loc.  cit.)  that  Hnseed  oil  thickened 
at  260°  C.  for  28-60  hours  contained  50  per  cent,  of  an  oil  of 
high  molecular  weight  insoluble  in  acetone.  An  examination 
of  the  two  portions  showed  that,  in  addition  to  polymeriza- 
tion, there  was  strong  indication  of  alteration  in  the  position 
of  the  double  linkages  of  the  mixed  glycerides  of  which  the  oil 
is  composed.  The  most  unsaturated  acids  are  polymerized 
first.  The  polymerized  glycerides  on  being  transformed 
into  the  methyl-esters  by  sodium  methylate  yield  mono- 
molecular  esters  of  acids  which  are  not  identical  with  the 
normal  acids  in  the  oil  glycerides.  The  drying  power  of 
the  polymerized  oil  is  much  slower  than  that  of  linseed  oil 
and  the  viscosity  varies  according  to  the  conditions  of 
heating.  lyceds  (/.  5.  C.  /.,  1894)  gives  the  results  of  the 
examination  of  a  number  of  lithographic  varnishes. 


Oxidized 

S.G.  at  15°  C. 

s.v. 

I.V. 

acids. 

Hexabromides 

Raw  linseed  oil 

o*'932i 

194-8 

169-0 

0-3  % 

24*17% 

Tint  varnish 

. .  0-9584 

I97'5 

1130 

1-5  % 

Thin  varnish 

0-9661 

196-9 

100-0 

2-5  % 

2-0 

Middle  varnish 

0-9721 

197*5 

91-0 

4-2 

o'95 

Strong  varnish 

.  .  0-9741 

190-9 

86-0 

6-5 

Burnt  thin  varnish 

.  .  0-9675 

195*5 

927 

0-85 

0-0 

(Burnt  thin  varnish  is  obtained  by  heating  the  oil  to  its  flash  point 
and  allowing  it  to  burn  quietly  with  constant  stirring). 

From  the  above  figures  it  is  evident  that  polymerization 
has  occurred  which  may  be  expressed  by  the  following 
scheme,  roughly  indicating  the  changes  in  the  orientation  of 
the  linkages  of  the  unsaturated  carbon  atoms. 

/An  /A'li  A'liv 

Glyceryl^Aiv       Glyceryl^A'iv    =  A'iv_Glyceryl. 
^Ayi  A'vi  A'vi  / 

An  and  A'n  =  oleic  acids 
Aiv  and  A'lv  =  linolic  acids 
Avi  and  A'vi  =  linolenic  acids 

The  incorporation  of  thickened  linseed  and  other  oils  into 
varnishes  and  paints  gives  a  coating  of  improved  protective 
power  against  corrosion  (M.  Toch,  /.  5.  C.  1915,  j^,  592), 
and  Friend  has  shown  that  polymerized  oils  in  paints  used 
on  iron  have  the  same  property.    The  formation  of  ring 


DRYING  OILS 


67 


complexes  consequent  on  polymerization  presumably  retard 
the  breaking  down  of  peroxides  or  oxides  of  the  glycerides 
on  exposure  to  atmospheric  agents. 

Theories  of  Driers. — The  drying  of  oils  is  greatly 
accelerated  by  the  presence  of  small  quantities  of  metals, 
metallic  oxides,  and  salts,  e.g.  linseed  oil  which  ordinarily 
takes  3  days  to  dry  will  dry  in  5-8  hours  in  the  presence  of 
5  per  cent,  of  dissolved  lead.  Many  other  metals  show  a 
similar  accelerating  action.  Fokin  {Seifen.  Zeit,,  j^,  821) 
places  metals  in  the  order  given  in  the  list : — 

Co,  Mn,  Cr,  Ni,  Fe,  Pt,  Pd,  Pb,  Ca,  Ba,  Bi,  Hg,  U,  Zn. 

He  states  that  the  velocity  of  drying  increases  with  the 
cube  root  of  the  concentration  of  the  catalyst ;  a  statement 
which  must  be  received  with  caution  because  drying  is  not 
solely  oxidation.  lyippert  {Zeitsch.  angew,  Chem,,  1898,  77, 
412),  Weger  {Chem.  Rev.  Fett.  u.  Harz.  Ind.,  1899,  4,  301),  and 
Kissling  (Z.  angew.  Chem.,  1891,  395)  showed  that  the 
drying  "  was  virtually  a  process  of  autoxidation  and  that 
linseed  oil  cotdd  absorb  or  combine  with  more  than 
20  per  cent,  of  its  weight  of  oxygen.  The  term  "  autoxi- 
dation is  given  to  those  processes  of  combustion  in 
oxygen  or  air  which  take  place  at  the  ordinary  temperature 
and  proceed  with  a  slow  but  measurable  velocity.  The 
reaction  velocity  of  the  oxidation  of  linseed  oil  was  examined 
by  Genthe  {loc.  cit.)  who  found  that  the  increase  of  weight- 
time  curve  showed  the  sinuous  character  of  an  autocatalytic 
reaction  (Figs.  14  and  15).  From  the  nature  of  this  curve 
it  must  be  assumed  that  some  intermediary  product  is 
formed  which  exerts  a  catalytic  function  on  the  oxidation 
of  the  oil.  This  intermediary  is  a  peroxide  (Engler  and 
Weissberg,  Chem.  Zeit.,  1903,  ^7,  1196).  The  formation 
of  the  intermediary  is  accelerated  by  light  (Genthe,  loc.  cit.). 
An  equation  corresponding  with  the  curves  obtained  by 
Genthe  is  of  the  following  form  : — 

dxjdt  =:K(a  —x)  [h  -\-x) 


a  and  b  are  the  initial  concentrations  of  linseed  oil  and  the 


68   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


autocatalyst  and  x  is  the  amount  of  linseed  oil  oxidized  ; 
therefore  a  quantitative  relationship  must  exist  between 
the  linseed  oil  oxidized  and  the  autocatalyst  generated. 

Mackey  and  Ingle  (/.  5.  C,  /.,  1917,  j6,  319)  classify  metals 
in  the  following  order  of  descending  powers  of  drying  : — 

Co,  Mn,  Ce,  Pb,  Cr,  Fe,  U,  Na,  Ag,  Zn,  Hg,  and  AL 

The  method  employed  is  novel :   cotton  wool  soaked  in 


6q 


Fig.  14. — Oxygen  Absorption  of  Oils. 

A.  Linseed  Oil 

B.  „         +1  %  Lead  Resinate  j 

C.  +1  %  Manganese  Oleate  I  In  the  dark. 

D.  „      ,,  +1  %  Benzoyl  Peroxide  ) 

F. '  +1"%  Lead  Okate}      '^'^^^^^  daylight. 

G.  Previously  exposed  to  light  from  mercury  lamp. 

linseed  oil  containing  the  metal  to  be  classified  was  placed  in 
a  cloth  oil-tester  {J.  S.  C.  /.,  1896, 16,  40)  and  the  time  taken 
to  attain  a  temperature  of  200"^  C.  was  noted.  The  great  rise 
in  temperature  during  the  oxidation  and  drying  of  an  oil 
shows  the  importance  of  preventing  the  accumulation  of 
oiled  waste  rag  in  a  factory  where  vegetable  oils  are  used. 
The  usual  method  of  testing  of  drying  power  is  that  of  the 
craftsman  who  fixes  the  time  when  the  film  becomes  satis- 
factorily dry  to  his  finger,  which  condition  is  preceded  by  a 


DRYING  OILS 


69 


"  dust  dry  period  (Davidson,  The  Setting  Value  of  Linseed 
Oil/'  Proc.  P.  and  V .  Soc).  Rough  though  the  method  is, 
with  a  personal  error  of  half  an  hour  and  a  dependence  on 
temperature  and  atmospheric  conditions,  it  is  satisfactory 
for  practical  purposes  in  the  hand  of  a  skilled  worker.  The 
results  in  the  gain-in-weight  trials  on  oxidation  are  to  be 
considered  as  deciding  broadly  between  drying  and  semi-dry- 
ing oils  or  between  metal  and  metal  as  driers/'  but  in 
practice  the  rate  of  oxidation  is  not  sufficient  to  establish 
the  superiority  of  any  metal. 


Fig.  15. — The  Drying  of  Linseed  Oil  (diffused  daylight.) 

Manganese  is  considered  by  Ingle  to  be  less  satisfactory 
than  lead,  although  its  oxidizing  power  is  greater,  and  his 
opinion  has  much  to  recommend  it  from  general  practice. 
From  the  lists  given  above  it  is  evident  that  a  metal,  which 
can  form  more  than  one  oxide,  acts  as  a  drier  or  oxygen 
carrier  when  it  is  in  an  oil-soluble  form,  provided  that  the 
salts  of  the  lower  oxides  are  more  stable  than  the  salts  of  the 

*  m  is  a  constant  which  is  proportional  to  the  initial  concentration  ol 
the  auto-catalyser  present. 


70   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


higher  oxides.  It  has  been  stated  that  the  more  oxides  a 
metal  can  form  the  greater  will  be  its  catalytic  activity. 
The  metallic  driers  are  catalysts,  and  the  term  catalysis  as 
defined  by  Henderson  Catalysis  in  Industrial  Chemistry/' 
1 91 9)  is  more  generally  used  to  designate  those  chemical 
changes  of  which  the  progress  is  modified  by  the  presence 
of  a  foreign  substance  :  the  agent  which  produces  the  effect 
is  called  a  catalyst.  The  theories  advanced  to  explain  the 
mechanism  of  catalysts  fall  into  two  classes  :  [a)  chemical 
and  {b)  physical. 

The  chemical  theories  of  drying  depending  on  the  forma- 
tion and  decomposition  of  unstable  intermediate  products  are 
the  most  favoured,  although  the  importance  of  the  physical 
aspect  is  growing.  On  Engler's  hypothesis  of  catalytic 
oxidation  {Ber.,  1897,  jo,  1669),  oxygen  (actor)  oxidizes 
the  metal  of  the  drier  (inductor),  which  in  turn  oxidizes  the 
oil  (acceptor).  In  the  description  of  the  general  properties 
of  linseed  oil  it  was  pointed  out  that  molecular  oxygen  was 
absorbed  at  the  double  linkages  with  the  formation  of 
peroxides.  The  peroxides  are  components  of  linoxyn," 
the  oxidized  skin  of  the  oil.  It  was  pointed  out  that  these 
peroxides  may  undergo  transformation  into  oxides  or  yield 
ketones  or  hydroxyketones  (Fahrion,  loc,  cit.)  ;  moreover, 
the  molecule  may  be  ruptured  in  the  presence  of  hydrolytic 
agents  to  yield  acids  and  aldehydes  (probably  polymerized 
with  aldol  condensation)  after  the  fashion  of  the  decom- 
position of  ozonides  described  by  Harries  and  others. 

X.CH^CH  :  CH— CH2Y  +02  =  X— CHa—CH :  CH.CHgY 

\/ 

XCH2CO.CH,.CH2Y  (Ketone)  +  O 
/7  \/  (to  oxidize  oil)  A 

/  O 

X.CH^.CHiCH.CH^Y^XCH^— CHOH.CO.CH2Y  (hydroxyketone :  the  hydroxy 
\/  I  ketones  lose  water  and  form 

O2  j  lactones  and  lactides  (Green, 

^    Aldehyde.        Aldehyde.  Ber.,  1909,  42,  3759). 

X.CH2.CHO  +  OHC.CH2Y      (Ingle,  /.  5.  C.  J.,  1913. 32,  639). 

From  the  above  it  is  evident  that  the  peroxide  will  not  be 
the  final  product  of  the  oxidation  of  linseed  oil,  and  from 
Genthe's  curve  with  its  equation  the  intermediate  peroxide 


DRYING  OILS 


71 


is  only  a  stage  in  the  drying  process.  From  an  investigation 
of  the  action  of  cerium  salts  on  Chinese  wood  oil  (Morrell, 
/.  Chem.  Soc,  1918,  iij,  iii)  it  is  evident  that  only  half 
the  amount  of  oxygen  is  taken  up  by  the  oil  as  would  be 
expected  from  the  number  of  double  linkages  and  that  it 
is  only  after  long  exposure  to  oxygen  that  cerium  tungate 
absorbs  an  amount  of  oxygen  corresponding  to  the  peroxida- 
tion of  all  the  double  linkages.  The  acid  isolated  was  a 
peroxide  which  slowly  polymerized. 

XCH  :  CH.Z-CH  :  CH.Y+02=X.CH  :  CH-Z.CH.CH.Y 

\/ 
O2 

->  X.CH.CH-Z.CH.CH.Y 

\/ 
O2 

X.CH.CH-Z.CH.CH.Y 

\/ 
O2 

It  is  possible  that  cobalt  and  manganese  may  produce 
superoxidized  oil  in  which  all  the  double  linkages  are  attacked, 
and  such  a  condition  would  not  favour  durability  because 
of  the  possibility  of  degradation  into  simpler  molecules. 

It  must  be  again  pointed  out  that  the  oxygen  absorption 
is  not  the  only  measure  of  the  drying  qualities  of  an  oil  as 
instanced  by  the  inferiority  of  drying  of  the  ethyl  esters  of 
linseed  oil  or  the  free  acids  or  the  ethyl  ester  of  j8-elaeostearic 
acid  (from  tung  oil)  (Morrell,  /.  Chem.  Soc,  1912,  loi,  2082) 
compared  with  their  respective  glycerides.  The  glyceryl 
radicle  probably  plays  a  part  in  the  setting,  oxidation 
the  polymerization.  To  return  to  the  function  of  the 
metal :  the  metallic  elements  act  as  oxygen  carriers  or  as 
pseudocatalysers  serving  to  stabilize  or  assist  in  the  formation 
of  the  autocatalytic  peroxide,  and  very  small  amounts 
are  able  to  produce  the  drying  of  large  quantities  of  oil. 
Many  theories  have  been  put  forward  to  explain  the  action 
of  the  metal  and  much  work  has  been  done  by  many  investi- 
gators, but  none  are  entirely  satisfactory  owing  to  the 

Note. — It  has  been  found  that  the  percentage  of  Hnoxyn  in  an  oil  film 
is  higher  if  the  oil  has  been  previously  polymerized  by  heat. — (R.  S.  M.) 


72   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


difficulty  of  investigating  the  products  of  the  reaction.  Ingle 
(/.  5.  C.  /.,  1917,^7,319),  after  reviewing  earher  work  on  the 
subject,  holds  the  view  that  in  boiled  oil  litharge  and  linseed 
oil  react  to  form  glyceryl  plumbolinolenate  and  linolate 
(there  is  general  agreement  that  the  glyceryl  radicle  is  unacted 
on  in  the  drying  process). 

C3H5(OL)3+2PbO=Pb02.C3H50L+Pb(OL)2 

LO.C3H5/     >Pb+0,=LO~C3H5/  >Pb< 

LO.  LOv  yO 

>Pb+02=  >Pb< 

yOs.        .0    CH—                    /O.  O— CH 
LO.C3H5<    >Pb<     +11        ^  LO.C3H5<    >Pb  +  |  | 

^O^      \0     CH—  O— CH 

(oil)                             1^  (oxidized  oil) 
LOv       yO    CH—          LO.  O— CH— 

>Pb<    +11  >Pb+  I  I 

LO^  CH—  1.0^        O— CH  — 

(oil)  (oxidized  oil) 

In  the  oxidation  of  cerium  tungate  (Morrell,  loc.  cit,)  the 
process  of  oxidation  of  the  salt  was  shown  to  be  represented 
by  the  scheme  : — 

O— CHY 

2CeX3->CeoOX6->Ce20(X02)6+oil'^2CeX3+  |      |       (oxidized  oil) 

O— CHY 

The  cerous  salt  changes  first  to  eerie  salt  which  facilitates 
the  formation  of  peroxides  of  the  acid. 

The  form  in  which  the  catalysts  are  used  varies  with  the 
requirements,  but  finely  divided  metals  (except  iron)  are 
very  rarely  used.  (I^ivache,  Compt,  Rend,,  1883,  g6,  250 
and  102,  1169.) 

The  metallic  oxides  and  salts,  especially  of  the  drying 
oils,  acids,  and  resinates,  are  generally  employed.  The  most 
important  inorganic  driers  are  litharge,  red  lead,  and  lead 
borate  or  the  oxides,  borate  or  sulphate  of  manganese. 
Among  the  salts  of  organic  acids  are  the  "  linoleates " 
of  lead,  cobalt,  and  manganese,  lead  acetate  and  manganese 
oxalate.  Iron  in  the  free  state  or  in  Prussian  blue  is  used 
in  the  patent  leather  industry. 

The  physical  theory  of  catalysis  seeks  to  explain  the 
phenomena  as  due  to  the   condensation  or  increase  in 


DRYING  OILS 


73 


concentration  of  the  reacting  substances  at  the  surface  of 
the  catalyst,  such  increase  in  concentration  being  due  to  the 
action  of  capillary  forces  (Henderson,  Industrial  Catalysis," 
igig).  This  aspect  has  been  neglected  in  the  consideration 
of  paint  and  varnish  drying  problems  {British  Association 
Reports,  1920).  The  influence  of  surface  phenomena  is 
of  great  importance  and  the  activity  of  the  metallic  driers 
is  due  partly  to  chemical  and  physical  causes.  For  a 
discussion  of  the  rival  theories  reference  may  be  made  to 
Mellor's  Chemical  Statics  and  Dynamics  "  ;  and  Lewis,  A 
System  of  Physical  Chemistry/'  Vol.  i ;  also  E.  Rideal, 
Catalysis  in  Theory  and  Practice,"  1919.*  It  must  be 
remembered  that  substances  of  a  colloid  nature  are  under  con- 
sideration in  paints,  varnishes,  and  oils,  and  the  activity  of 
metals  in  solutions  containing  colloids  is  more  active  than  in 
aqueous  solutions.  If  the  surface  tension  to  air  of  a  lead 
drying  oil  is  lower  than  that  of  the  oil  (there  is  reason  to 
believe  that  this  is  so)  the  surface  concentration  of  the 
lead  would  be  increased.  A  more  careful  study  of  lyivache's 
method  by  drying  in  the  presence  of  finely  divided  lead  would 
establish  a  connection  with  the  phenomena  observed  when 
unsaturated  oils  are  reduced  with  hydrogen  in  the  presence  of 
metallic  catalysts.  It  is  for  the  chemist  to  decide  from  the 
examination  of  the  products  of  oxidation  the  chemical 
changes  which  have  occurred.  It  would  appear  that  the 
activity  of  driers  is  to  be  attributed  to  causes  put  forward 
by  both  theories  taken  together. 

Methods  of  Testing  the  Commoner  Drying  Oils. — 
It  is  impossible  to  go  into  the  details  of  the  testing  of  the 
commoner  drying  oils  in  this  general  review  of  their  proper- 
ties. Reference  must  be  made  to  the  many  text-books  on 
the  subject.    A  few  English  publications  may  be  mentioned. 

lycwkowitsch,  Oils,  Fats,  and  Waxes/'  5th  edition. 
Macmillan. 

Fryer  and  Weston,  Oils,  Fats,  and  Waxes."  Camb. 
Univ.  Press.  1917. 

*  Morrell,  "Catalysis  applied  to  the  Oxidation  of  Oils,"  /.  S.  C. 
1920,  J9,  153T. 


74   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Ingle,     Oils,  Resins,  and  Paints/'    Griffin.  1915. 

A.  C.  Wright, Analysis  of  Oils/'  Crosby  lyockwood.  1903. 

The  most  recent  account  and  fullest  in  detail  is  given  in 
Fryer  and  Weston's  work,  Part  2,  to  which  the  reader  must 
be  referred.  Only  those  methods  which  concern  drying  oils 
will  be  indicated  here.  Usually  it  is  advisable  to  determine : — 

(1)  Specific  gravity  at  15*5  C. 

(2)  Iodine  value. 

(3)  Saponification  value. 

(4)  Acid  value. 

(5)  Insoluble  bromide  value  of  the  oils  or  their  corre- 
sponding acids. 

(6)  The  drying  times  of  the  oil  against  a  standard 
linseed  oil. 

(1)  Specific  Gravity. — With  the  exception  of  tung  oil 
(0*940),  all  the  drying  oils  have  a  specific  gravity  between 
0*925  and  0-933. 

(2)  Iodine  Value. — ^This  is  a  valuable  guide  as  to  the  nature 
of  the  oil  as  will  be  evident  from  the  following  figures  : — 

Perilla  oil :  205-206. 
Linseed  oil :  170-200. 

Tung  oil :  160-170,  distinctive  odour ;  somewhat  un- 
pleasant. 

Candlenut  oil :  164. 

Hemp  oil :  148,  oil  generally  of  a  greenish  colour. 
Walnut  oil :  140-150,  characteristic  odour  of  walnuts. 
Poppy  oil :  130-140. 

Soya  bean  oil :  128-135,  a  slight  distinctive  odour  and 
high  content  of  saturated  acids,  (13-15  percent.),  (de  Waele). 
Pararubber  seed  oil :  138. 
Sunflower  oil :  125-133. 

Menhaden  oil :  160-182,  fishy  odour.  Fish  oils  are 
especially  recognized  by  the  insolubility  of  their  brominated 
products  in  a  mixture  of  acetic  acid  and  carbon  tetrachloride. 

If  the  oil  is  a  raw  oil,  adulteration  with  non-drying  oils 
would  be  indicated  by  the  iodine  value,  but  a  polymerized 
linseed  oil  would  give  values  much  lower,  so  that  other  tests 
are  necessary. 


DRYING  OILS 


75 


(3)  Saponification  Value. — This  is  generally  186-196. 
A  low  saponification  value  would  indicate  adulteration  with 
petroleum,  rosin  oil,  and  even  rosin. 

(4)  Acid  Value. — This  is  for  most  oils,  0-2.  Generally 
the  acid  value  is  a  guide  as  to  the  grade  of  an  oil  and  the 
care  taken  in  its  extraction.  Polymerized  oils  have  a  higher 
acidity  and  the  presence  of  rosin  raises  the  value  con- 
siderably. 

(5)  The  Insoluble  Bromide  Value. — This  is  an  important 
determination  often  overlooked  because  of  the  time  required 
in  carrying  it  out.  The  principle  is  the  estimation  of  the 
ether-insoluble  bromides  obtained  from  the  oil  acids  or 
glycerides.  From  the  values  it  is  possible  to  form  a  fair 
idea  of  the  proportion  of  linseed  or  menhaden  oil  present  in 
a  jtnixture.  Tung  oil  gives  no  ether  insoluble  bromides  nor 
do  the  lithographic  oils.  I^inseed  oil  gives  32-38  per  cent.  ; 
candlenut  oil,  ii'o ;  menhaden  oil,  63-64.  (Steele  and 
Washburn,  /.  Ind  Eng.  Chem.,  1902,  12,  52.) 

(6)  Time  of  Drying. — The  drying  times  of  a  lead- 
manganese  drying  oil  against  a  similar  drying  oil  made  from 
linseed  oil  is  a  necessary  test  to  be  made.  The  nature  of  the 
film  is  of  importance. 

The  solidification  test  for  tung  oil  (Browne's  test)  is  the 
quickest  way  of  characterizing  the  oil,  because  the  presence 
of  5-10  per  cent,  of  foreign  oils  can  be  detected  by  its  means. 
The  principle  of  the  method  is  the  coagulation  at  282""  C.  in 
12  minutes  of  a  small  quantity  of  the  oil  which,  w^hen  solid 
at  that  temperature,  will  show  a  clean  cut  when  stabbed 
with  a  steel  spatula. 

It  is  advisable  to  test  oils  for  added  rosin  or  rosin  in  the 
form  of  resinates  which  improve  the  drying.  It  can  be 
detected  by  the  Iviebermann-Storch  Reaction  (Fryer  and 
Weston,  Part  2,  222).  A  high  acidity  would  indicate  free 
rosin  added,  otherwise  the  presence  of  rosin  is  due  to  metallic 
resinates  which  can  be  detected  by  examination  of  the  ash. 

For  details  as  to  lyinseed  Oil  Standards  reference  must  be 
made  to  :     Some  Technical  Methods  of  Testing  lyinseed  Oil/' 

.  *  Analyst,  1912,  jy,  410. 


76   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Bureau  of  Standards,  Washington,.  U.S.A.,  1916,  page  11 ; 

Reports  of  the  American  Society  for  Testing  Materials," 
1915,  417-427. 

There  are  for  each  oil  characteristic  properties,  for  the  details 
of  which  references  must  be  made  to  the  authorities  quoted. 
Sufficient  has  been  given  to  decide  whether  the  oil  is  a  genuine 
oil  by  its  sp.  gr.  and  saponification  value  :  a  genuine  drying  oil 
can  be  recognized  by  the  iodine  value  and  drying  time  as  well 
as  the  methods  of  detection  of  the  common  adulterants,  e.g. 
rosin  and  petroleum.  To  prove  the  presence  of  foreign  oils, 
which  may  be  suspected,  it  will  be  sufficient  to  carry  out  the 
six  main  classes  of  operations  outlined  and  to  apply  from 
the  information  gained  the  characteristic  tests  given  imder 
the  individual  oils.  The  difficulty  of  estimating  the  thickened 
oils  in  a  mixture,  especially  wood  oil,  has  up  to  the  present 
not  been  entirely  overcome,  although  thickened  oils  are  only 
partially  soluble  (50  per  cent.)  in  acetone,  and  a  very  fair  con- 
clusion can  be  drawn  by  that  method  of  separation  (Morrell, 
J.  5.  C.  /.,  1915,  j^,  105).  Tung  oil  may  be  detected  in  the 
absence  of  rosin,  turpentine,  and  oxidized  oils,  by  giving  a 
port  wine  colour  with  iodine  monochloride  (Wijs'  method)  due 
to  the  liberation  of  iodine  consequent  on  the  formation 
of  HCl  by  the  action  ICl  on  the  oil. 


Part  III.— RESINS  AND  PITCHES 


Resins 

The  resins  are  essentially  of  vegetable  origin,  being 
exudations  of  trees  of  many  different  genera  and  species. 
It  is  only  recently  that  artificial  resins  derived  from  coal 
tar  prodticts,  e.g.  Bakelite,  Cumarone,  and  Paraindene 
resins,  have  been  used  as  substitutes,  although  for  oil 
varnishes  the  natural  resins  are  still  preferred. 

Commercially  the  resins  belong  to  the  class  of  gums  which 
are  for  the  most  part  produced  as  exudations  of  trees.  The 
gums,  when  incorporated  with  liquids,  form  more  or  less 
viscous  fluids  which  on  drying  give  films  of  varying  protective 
character.  They  all  show  the  properties  of  colloids,  usually 
forming  emulsoid  solutions  with  water,  oil  or  other  media. 
Some  are  completely  soluble  in  water,  others  swell  up  to 
give  jellies,  whilst  others  are  unacted  on.  The  colloid 
properties  of  gums  are  in  many  respects  similar  to  those  of 
glue  or  gelatin,  which  is  a  typical  colloid  containing  nitrogen, 
and  is  of  animal  origin.  The  resins "  can  be  readily 
distinguished  from  the  true  gums  "  by  the  following  simple 
methods  : — 

(1)  When  a  resin  is  held  in  a  flame  it  takes  fire  and  burns 
with  a  smoky  flame  giving  off  an  aromatic  odour.  A  gum 
similarly  treated  chars  and  smells  of  burnt  sugar. 

(2)  A  resin  placed  in  water  is  unaltered,  whereas  a  gum 
dissolves  or  forms  a  jelly. 

(3)  When  a  resin  is  allowed  to  stand  in  methylated  spirit 
or  in  turpentine  it  disintegrates  and  dissolves  either  partially 
or  completely.    A  true  gum  is  unacted  on  by  these  solvents. 

77 


78    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Selected  Reports  from  the  Scientific  and  Technical  Depart- 
ment of  the  Imperial  Institute/'  Part  3,  Gums  and  Resins, 
No.  63,  1909.) 

Although  only  the  resins  are  the  important  ingredients 
of  varnishes  a  passing  reference  to  several  of  the  true  gums 
is  advisable. 

Gum  Arabic  is  obtained  from  varieties  of  acacia  and  is 
known  in  the  trade  under  many  forms,  e.g.  Turkey,  White, 
Sennar,  Kurachee,  Mogador,  Wattle,  and  Senegal.  These 
forms  are  all  more  or  less  soluble  in  water  and  swell  up  in 
that  medium.  They  are  insoluble  in  alcohol  and  on  boiling 
with  dilute  sulphuric  acid  give  a  sugar  (pentose  or  hexose), 
thus  showing  their  carbohydrate  nature.  The  best  qualities 
are  used  for  pharmaceutical,  confectionery,  and  other 
purposes,  whilst  the  commoner  qualities  are  in  demand  in 
textile  industries,  as  a  binding  material  for  artists'  water- 
colours  and  for  mucilages. 

Gum  Tragacanth  is  obtained  from  various  species  of 
Astragalus,  indigenous  to  Greece,  Asia  Minor,  Syria,  and 
Persia.  The  gum  flows  freely  from  the  stems  when  they  are 
wounded.  It  is  softer  than  acacia  and  cherrj'-  gums.  Like 
gum  arable  it  gives  a  viscous  fluid  with  water  and  is  hydro- 
lysed  by  dilute  sulphuric  acid  to  yield  sugars.  It  is  used 
as  a  thickener  in  calico  printing  and  as  a  vehicle  in  many 
pharmaceutical  preparations. 

Reference  must  be  made  to  the  Balsams  which  are 
solutions  or  emulsions  of  resins  in  oil  esters,  e.g.  esters  of 
cinnamic  or  benzoic  acids.  The  well-known  Canada  balsam 
contains  18  per  cent,  of  oil  esters,  and  Copaiva  balsam 
contains  as  much  as  60  per  cent,  oil  esters.  Other  important 
balsams  are  Peru  balsam  and  Mecca  balsam.  The  balsams 
may  be  thick  or  thin  fluids  possessing  usually  a  characteristic 
odour. 

The  Resins  are  exudations  of  trees  (the  vegetable  origin 
of  lac  is  disputed) .  The  maj  ority  are  used  in  varnish  making. 
About  five-sevenths  of  the  total  imports  of  varnish  gums  or 
resins  into  the  United  Kingdom  are  obtained  from  Imperial 
sources.    In  1908  the  imports  into  the  United  Kingdom  were 


RESINS  AND  PITCHES 


79 


valued  at  ^2,803,535  ;  they  included  shellac  from  India, 
kauri  from  New  Zealand,  copal  from  British  West  Africa, 
animi  from  Zanzibar,  and  dammar  and  copal  from  Singapore. 
Most  of  the  Zanzibar  animi  is  collected  in  German  East 
Africa  and  a  large  part  of  the  Singapore  copal  comes  from  the 
Dutch  East  India  possessions.  The  resins  are  more  or  less 
hard  (elemi  is  soft),  friable  or  brittle,  transparent  or  lustrous. 

They  are  insoluble  in  water  ;  some  are  soluble  in  alcohol, 
ether,  or  benzol ;  others  require  strong  fusion  before  they 
can  be  incorporated  with  solvents  used  in  varnish  making. 
The  researches  of  Tschirch  and  his  pupils  have  thrown  light 
on  the  composition  of  the  resins,  but  a  fuller  investigation 
is  necessary  and  a  careful  comparison  of  the  components 
and  of  the  decomposition  products  is  very  desirable.  Gener- 
ally the  resins  contain  acids  Resinolic  Acids,"  Tschirch), 
e.g.  rosin  (colophony)  contains  abietic  acid,  amber  contains 
succinoabietic  acid  from  which  succinic  acid  can  be  isolated  ; 
dammar  yields  dammarolic  acid  CseHgoOg  ;  Zanzibar  copal 
contains  trachylolic  acid  CseHsgOg.  Some  acids  are  mono- 
basic, others  dibasic. 

An  important  component  ate  the  rescues  (Tschirch), 
which  are  oxygenated  bodies,  neither  alcohols,  esters,  nor 
aldehydes.  They  are  insoluble  in  alkalies  and  their  inertness 
renders  their  presence  in  resins  of  great  importance  for 
varnish  making.  It  has  been  suggested  that  the  rescue 
content  is  of  value  in  estimating  the  quality  of  a  varnish 
gum. 

Elemi  contains  elemic  acid  €35114504,  and  amyrin  rescue 
C25H42O. 

Dammar  contains  dammarolic  acid  CsgHsoOg,  and 
a-resene  C11H17O,  jS-resene  C31H52O. 

Mastic  contains  an  acid  C20H32O2,  and  masticin  rescue 
C20H32O. 

Lac  contains  an  acid,  aleuritinic  acid  (C13H26O4),  and  a 
rescue . 

From  the  resin  esters  (resines),  resin  alcohols  (resinols 
and  resinotannols)  have  been  isolated  by  Tschirch  and  his 
collaborators. 


8o    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Amber  contains  a  succinoresinol  C12H20O  :  m.p.  275°  C. 

Pine  resin  contains  a  pinoresinol  CioHxgOe  :  m.p.  80-90°  C. 

Elemi  contains  a  and  ^-amyrin  C30H49OH. 

Lac  contains  resinotannol ester  of  aleuritinic  acid  C]  3H26O4. 

Sumatra  benzoin  contains  sumaresinotannol  Ci8Hi903(OH) 
(which  yields  picric  acid  with  nitric  acid  and  pyrocatechuic 
acid  on  alkali  fusion). 

From  Tschirch's  investigation  it  is  evident  that  the  resins 
contain  resinolic  acids,  resin  esters,  and  resenes. 

The  most  important  characteristics  of  the  resins  are 
colour,  hardness,  lustre,  and  specific  gravity.  Some  resins, 
e.g.  mastic,  occur  in  droplike  pieces  as  the  resin  flows  from 
the  tree,  others  in  short  cylindrical  pieces  agglomerated 
together  as  in  sandarac,  and  yet  others  in  large  irregular- 
shaped  masses  as  copal  gums.  Lac  may  appear  in  stick 
form  from  the  twig  on  which  it  has  been  produced  by  the  lac 
insect.  In  the  trade  the  resins  often  appear  in  artificial 
forms  due  to  previous  treatment  in  the  country  of  the 
source,  e.g.  shellac  in  thin  plates,  dragon's  blood  in  sticks. 
The  surface  of  a  resin  is  often  characteristic.  Weathering 
of  the  surface  produces  a  goose  skin  appearance  as  shown  in 
Zanzibar  copal. 

According  to  Wiesner  (''Die  Rohstoffe  des  Pflanzen- 
reiches'')  not  one  of  the  resins  from  plants  is  a  chemical 
individual  but  a  complex  mixture.  Mastic,  sandarac,  and 
dammar  appear  to  be  homogeneous  :  crystals  are  sometimes 
found  in  a  matrix  of  resin  as  in  elemi  and  Fichtenharz.  The 
fracture  of  the  gums  is  characteristic  and  conchoidal,  some- 
times lustrous  or  dull,  e.g.  Zanzibar  copal  shows  dull  and 
lustrous  alternations.  Benzoin,  yellow  accroides  and  others 
show  almond  structure  with  agglomerates  of  coarsely 
rounded  masses. 

The  colour  is  very  varied  :  from  colourless  to  the  red 
of  dragon's  blood,  the  yellow  of  gamboge,  and  the  black  of 
some  varieties  of  rosin  (colophonium) .  Generally  the  colour 
is  yellow  to  brown.  The  lustre  is  vitreous,  but  it  may  be 
waxy  as  in  the  so-called  almonds  of  benzoin.  Some  copals 
are  entirely  lustreless. 


RESINS  AND  PITCHES 


8i 


The  most  important  characteristics  are  hardness,  fusibility, 
and  solubility  in  solvents. 

Hardness, — According  to  Wiesner  {loc.  cit)  the  hardness 
of  the  copal  resins  lies  between  gypsum  and  rocksalt ;  only 
the  best  copals  are  harder  than  rocksalt  and  a  few,  e.g,  elemi, 
are  so  soft  that  they  can  be  worked  in  the  fingers.  The 
softer  varieties  have  low  melting  points ;  Siam  benzoin 
melts  at  75"^  C,  whilst  the  hardest  copals  (Zanzibar)  do  not 
melt  below  360°  C.  Generally  the  less  fusible  gums  give 
the  hardest  coatings.  Shellac  gives  a  hard  coherent  layer 
whilst  the  fusible  colophonium  is  easily  rubbed  up  by  the 
finger.    In  hardness  and  fusibility  there  is  a  wide  range. 

The  solubility  is  important  in  deciding  the  uses  of  the 
resin  :  some  resins  are  readily  soluble  in  methylated  spirit 
and  are  used  for  spirit  varnishes,  viz.  shellac,  sandarac, 
colophonium,  and  the  softer  varieties  of  copals.  Others 
such  as  dammar  and  mastic  are  soluble  in  turpentine. 
Ether,  benzol,  and  acetone  are  solvents  for  a  number  of  resins. 
Copals  are  generally  insoluble  in  vegetable  oils  until  they  are 
fused,  but  they  will  dissolve  easily  in  the  acids  obtained  from 
drying  oils  and  in  the  drying  oils  themselves  after  the  resins 
have  been  run."  In  petroleum  ether  they  are  insoluble 
or  sparingly  soluble.    (Compare  Table  of  Solubilities,  p.  87.) 

The  Formation  of  Resins  in  the  Plant. — The  resin 
secretion  cells  of  a  plant  may  be  stimulated  by  wounding 
to  give  a  flow ;  even  if  the  plants  show  no  such  secretion 
cells  normally,  these  cells  can  be  generated,  as  in  liquidambar, 
styrax  and  benzoin,  by  destroying  the  new  wood,  whereby 
the  flow  of  balsam  can  be  continued  for  years  if  the  wound 
is  enlarged  from  time  to  time.  (Tschirch,  Harze  u.  Harzbe- 
halter'';  Wiesner,  "Die  Rohstoffe  des  Pfanzenreiches " ; 
lyivache  and  Mcintosh,  ''The  Manufacture  of  Varnishes,'' 
Vols.  2  and  3  ;  Dieterich,  ''Analyse  der  Harze.'') 

Tschirch  favours  the  view  that  lac  is  a  secretion  from  the 
lac  insect  and  not  a  direct  exudation  of  a  tree  caused  by  the 
insect.  It  is  evident  that  the  production  of  resins  is  akin  to 
that  of  the  rubber  latex  referred  to  in  Part  I.  of  this  volume. 
It  is  chiefly  in  connection  with  the  oleoresin  from  species 
S.  6 


82   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


of  pine  that  the  production  of  resin  has  been  systematically 
studied.  The  output  of  turpentine  resin  is  large  compared 
with  other  resins.  Many  forms  of  copal  are  of  fossil  origin, 
whilst  the  softer  copals  from  living  trees  are  collected  by 
aborigines,  and  little  attention  is  paid  to  improving  the 
yield  or  extending  the  cultivation  of  the  parent  tree. 

Classification  of  the  Resins. — In  view  of  the  com- 
plexity of  chemical  composition  of  the  resins  and  in  spite  of 
the  investigations  of  Tschirch  and  his  pupils,  it  is  difhcult 
to  classify  them  so  that  differences  in  chemical  composition 
will  go  hand  in  hand  with  their  properties  which  are  of 
industrial  importance.  Dieterich  proposes  the  following 
classification  : — 

(1)  Resins  which  are  esters  of  the  aromatic  series,  either 
containing  free  acids  or  not,  e.g.  benzoin,  dragon's  blood, 
accroides.    These  resins  are  soluble  in  methylated  spirit. 

(2)  Resins  which  are  esters  of  special  resin  acids  with  or 
without  free  resin  acids :  turpentine  oleoresin,  mastic, 
elemi.  These  are  soluble  or  partly  soluble  in  methylated 
spirit  or  soluble  in  organic  solvents. 

(3)  Resins  which  are  free  acids  containing  rescues, 
e.g.  copals,  dammar,  sandarac,  and  colophonium. 

For  practical  purposes  the  scheme  adopted  in  Hurst's 
text-book  (2nd  edition)  is  more  useful,  viz.  : — 

{a)  Oil  varnish  resins,  (b)  spirit  varnish  resins,  {c)  resins 
soluble  in  special  organic  solvents. 

Unfortunately  there  is  considerable  overlapping  in  this 
simple  classification.  In  the  first  class  appear  the  copals, 
which  are  recent  or  fossil  resins  produced  in  prehistoric 
forests  or  dug  from  the  soil  in  what  is  still  a  forest  region. 

The  changes  which  have  occurred  in  the  secreted  or 
exuded  gums  are  not  understood.  Undoubtedly  there  has 
been  a  reduction  in  acidity  :  with  polymerization,  or  coagula- 
tion, oxidation  has  occurred  (the  rescues  are  inactive  oxides), 
but  moreover  with  such  a  mixture  of  components  in  a  colloid 
complex  it  is  unwise  to  speculate.  The  recent  copals, 
including  manila,  pontianak,  etc.,  are  not  in  demand  by  the 
makers  of  high-class  oil  varnishes,  but  their  softer  varieties 


RESINS  AND  PITCHES 


83 


are  soluble  in  methylated  spirit.  Rosin  (colophonium)  is 
often  a  component  of  cheap  oil  varnishes  when  rosin  is 
cheaper  than  copal,  and  especially  in  varnishes  containing 
China  wood  oil.  In  the  class  of  oil  varnish  resins  amber 
must  be  mentioned,  although  it  is  no  longer  used  because 
of  its  price.  Tschirch  and  de  Jong  {Arch.  Pharm.,  1915, 
290)  state  that  the  main  component  of  amber  (succinite) 
is  succinoresene,  which  is  the  cause  of  its  resistance  to  the 
action  of  reagents.  Amber  contains  two  acids,  succinoxy- 
abietic  acid  C19H29O2COOH  and  succinoabietic  acid 
C39H59COOH. 

The  second  class  includes  the  methylated  spirit  soluble 
resins,  lac,  sandarac,  mastic,  manila  copals,  rosin,  and 
accroides.  These  are  used  for  coatings,  giving  lustrous 
films  of  rather  a  brittle  character  and  slight  durability  for 
outside  wear.  lyac  is  undoubtedly  the  most  valuable  of 
its  class,  although  mastic  is  prized  as  a  picture  varnish 

megilp     when  dissolved  in  a  suitable  solvent. 

The  third  class  included  many  of  the  second  in  addition 
to  dammar  and  the  oil  varnish  resins  which  have  been 
fused  or  run.''  The  solvents  employed  may  be  turpentine, 
benzol,  acetone,  or  coal-tar  naphtha.  The  above  classifica- 
tion is  only  fairly  satisfactory,  but  it  is  impossible  to  adopt 
a  scheme  based  on  differences  in  chemical  composition  when 
in  practice  differences  of  physical  properties  are  of  greater 
importance.  In  the  short  summary  of  the  properties  of 
resins  the  special  characteristics  of  a  few  typical  members 
of  each  class  will  be  given. 

{a)  Oil  Varnish  Resins  :  Copals. — These  are  essentially 
fossil,  recent  fossil,  and  recent  resins.  Some  resins  classed 
as  copals  are  now  a  yearly  crop  and  are  used  in  oil  varnishes. 
At  present  the  bulk  of  the  copal  used  for  oil  varnishes  in 
this  country  is  of  fossil  origin,  obtained  from  Central  Africa 
(Congo),  Zanzibar  (Animi),  West  Africa  (Angola),  Manila  and 
the  East  Indies  and  New  Zealand  (Kauri).  The  supplies  of 
fossil  copals  are  limited  and  sooner  or  later  the  softer  copals 
obtained  from  living  trees  will  have  to  be  utilized.  It  is 
stated  that  the  supply  of  kauri  copal  will  last  for  forty  years 


84   RUBBER,  RESINS  PAINTS,  AND  VARNISHES 


at  the  present  rate  of  output  (U.S.  ''Commerce  Report," 
No.  281,  1915).  The  resin  is  fossil  from  Dammara  australis 
(a  species  of  New  Zealand  pine).  The  gum  obtained  from 
living  trees  is  known  as  young  kauri  and  is  softer  and  almost 
colourless.  Young  trees,  when  tapped,  yield  the  resin,  and  it  is 
not  uncommon  to  find  deposits  of  resin  in  old  trees.  For 
fresh  sources  of  copals  it  is  probable  that  the  belt  of  country 
extending  from  Madagascar  to  Sierra  lycone  and  possibly 
the  Gambia  will  be  the  most  promising.  The  fossil  East 
African  copals  (Zanzibar,  Madagascar,  Mozambique,  lyindi) 
are  highly  prized.  The  Zanzibar  animi  copal,  with  its 
peculiar  ''goose  skin''  appearance,  occurs  in  various  sized 
pieces  but  not  in  large  masses  like  kauri  and  other  gums. 
It  cannot  be  scratched  by  the  finger  nail :  its  melting  point 
is  240°-25o°  C,  and  its  ''  running  "  temperature  is  much 
higher.  Bottler  found  the  melting  point  of  some  samples 
as  high  as  340°-36o^  C.  (chapter  on  ''Varnish  Making"). 
The  East  African  fossil  gums  are  derived  from  a  species 
of  trachylobium  occurring  in  Madagascar,  in  German 
and  Portuguese  East  Africa,  and  probably  in  British  East 
Africa.  The  West  African  copals  (Red  Angola  and  Congo), 
are  probably  derived  from  Copaifera  mopane,  which 
occurs  in  the  great  forests  along  the  Zambesi  valley.  In 
Southern  Nigeria,  the  Gold  Coast,  and  Ashanti,  Daniella 
ohlonga  is  the  source,  and  in  Sierra  I^eone  and  French 
Guinea  Copaifera  guibourtiana.  Unfortunately  the  forests 
have  been  much  depleted  by  wasteful  tapping.  All  the 
copal  obtained  from  British  West  Africa  comes  from  living 
trees  and  is  not  a  fossil  resin.  In  Sierra  lycone  the  trees 
are  tapped  on  a  definite  system  which  consists  in  cutting 
out  portions  of  the  bark  about  2  to  3  inches  square  at  intervals 
of  about  9  inches  on  the  surface  of  the  stem  and  larger 
branches  of  the  tree.  The  process  resembles  tapping  for 
rubber  or  turpentine  resin,  but  it  is  more  difficult,  because 
rubber  latex  is  secreted  in  lactiferous  cells  lying  near  the 
surface,  while  the  resins  are  secreted  either  in  isolated  vesicles 
in  the  bark  or  in  resin  ducts  lying  in  the  bark  or  sap  wood  ; 
moreover,  the  trees  do  not  show  such  a  ''wound  response 


RESINS  AND  PITCHES 


85 


as  in  the  case  of  the  Para  rubber  tree,  which  gives  a  large 
increase  of  latex  on  second  tapping.  Restrictions  are  now 
placed  on  unsystematic  tapping  and  the  Colonial  Govern- 
ments are  endeavouring  to  increase  the  production  of  copal 
by  planting  trees  and  improving  the  yield  of  the  resin  (T. 
H.  Henry,  ''Some  Colonial  and  Indian  Resins,"  Proc, 
Paint  and  Varnish  Soc,  1913). 

The  Manila  copals  are  derived  from  Agathis  loranthifolia 
and  appear  on  the  market  as  Manila,  Borneo,  Macassar,  and 
Pontianak  copals.  The  trees  are  tapped  by  a  method 
resembling  the  box  system  used  for  turpentine  resin 
in  the  United  States.  In  Manila  long  strips  of  bark  are  cut 
from  the  trees  and  the  exuded  resin  is  collected  as  soon  as 
the  latter  has  dried  hard  :  the  same  method  is  used  in  the 
collection  of  dammar  gum.  The  softer  varieties  of  Manila 
copals  are  soluble  in  methylated  '  spirit,  while  the  hard 
varieties  are  used  by  some  varnish  makers,  although  they 
have  not  the  popularity  of  the  African  copals  and  the 
New  Zealand  kauri.  The  Brazilian  copals  are  not  in  great 
demand  because  of  their  softness. 

The  copal  resins  are  graded  according  to  hardness,  colour, 
fusibility,  ash  content,   acidity,  and  loss  of  weight  on 
running.''    The  following  scale  of  hardness  has  been 
suggested  by  Bottler  who  takes,  as  standard,  Zanzibar 
copal. 

Zanzibar  copal,  Mozambique,  lyindi.  Red  Angola,  Pebble, 
Sierra  lycone.  Yellow  Benguela,  White  Benguela,  Cameroon, 
Congo,  Manila,  White  Angola,  Kauri,  Sierra  lyCone  (new), 
Hymenea  [S.  America,  Brazil,  Demerara,  from  the  locust 
tree].    These  copals  are  in  order  of  descending  hardness. 

In  lustre  and  colour  the  copals  are  vitreous,  transparent, 
dull,  colourless  to  yellow,  reddish  yellow  and  black.  A 
vitreous  gum  is  preferred  to  a  dull  cloudy  resin.  In  fusibility 
the  hardest  copals  have  the  highest  melting  points.  Zanzibar 
melts  at  240°-25o°  C. :  West  African  copals  melt  at  120°- 
180°  C;  Sierra  I^eone,  I25°-I37°C. ;  Manila  and  Kauri, 
Ii5°-i40°  C. ;  Hymenea  copal  (fossil),  i8o°-20o''  C,  but  the 
softer  varieties  melt  below  115°  C.    The  fusibility  depends 


86    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


essentially  on  the  age,  the  new  varieties  fusing  at  lower 
temperatures.  In  view  of  the  complexity  of  composition 
no  fixed  temperature  can  be  assigned  to  any  variety.  It  must 
be  pointed  out  that  the  resins  first  soften  and  then  slowly 
liquefy  and  the  running  temperature  is  the  point  when 
suitable  fluidity  is  attained  which  may  or  may  not  coincide 
with  the  melting  point.  For  each  gum  there  is  a  temperature 
below  which  it  cannot  be  incorporated  with  linseed  oil. 

A  similar  case  might  be  quoted  in  the  melting  of  sulphur 
as  regards  the  necessary  fluidity.  Generally  the  softer 
copals  are  easier  to  work  in  every  sense  of  the  term. 

Acidity. — This  is  a  number  of  considerable  importance  in 
the  evaluation  of  copals.  If  a  resin  mixing  is  considered  from 
the  standpoint  of  a  colloid  the  acidity  factor  must  not  be 
neglected.  The  acidity  of  a  run  "  gum  is  not  the  same  as 
that  of  the  native  copal :  usually  it  is  reduced  to  half  its 
original  value.  A  high  percentage  of  esters  is  indicated 
by  high  saponification  value  (see  Table  of  Constants  of 
Resins). 


Constants  of  Resins. 


specific 

Melting  point. 

Acid 

Saponifi- 
cation 
value. 

Iodine 

gravity. 

Softens. 

Fuses. 

value. 

value. 

Kauri,  brown 

I-053 

90^  c. 

185°  c. 

93-0 

ii9'5 

fused 

63-0 

Manila,  hard 

1-065 

80° 

190°  c. 

72-8 

227-1 

90-6 

,,  soft 

i-o6o 

120^  c. 

145-2 

Dammar,  Batavian 

1-031 

100^  c. 

(20-35) 

47-0 

63*6 

Mastic 

1-057 

95°  C. 

50-70 

79-1 

64-4 

Rosin 

i-oy 

80°  C. 

100°  C. 

145-185 

168-2 

112-0 

Copal,  Zanzibar 

1-058 

360^  c. 

60-65 

92-4 

„  fused 

6i-o 

37'0 

126-8 

Sierra  Leone  (A) 

1-0645 

195°  c. 

84-6 

„  (B) 

i'o66 

200®  c. 

130-0 

„     Red  Angola  . . 

I -066 

90° 

I45°-3I5°C. 

129-0 

132*0 

63-0 

Benguela 

1-058 

65° 
50^ 

140^-160° 

134-0 

146-0 

60-5 

,,  Brazil 

1-053 

100° 

II2-0 

151*0 

59'o 

amber 

r-o8o 

280^-315° 

15-35 

87-0 

62-1 

,,  sandarac 

1-073 

145° 

95-160 

174-0 

160-0 

„    shellac  ..| 

i-ii3-j 

I-2I4  / 

63-0 

203-0 

8-24 

turpentine 

(oleo  resin) 

0-856 

130-  c. 

69-8 

143-6 

RESINS  AND  PITCHES  87 

Action  of  Solvents  on  Resins. 


Percentages  of  insoluble  matter  (Coffignier,  "Manuel  du  fabricant  de 
vernis,  gommes,  resines 


Turpen- 
tine. 

Alcohol. 

Ether. 

Benzol. 

Petrol  ether. 

Zanzibar  copal 

lOD'O 

85-9 

75'o 

88'o 

Insol. 

Benguela 

69-0 

16-5 

437 

65-6 

or  almost  insol. 

Kauri 

77*5 

7-0 

62-0 

67-0 

ft          tt  ft 

Red  Angola  ,, 

77-0 

71-0 

88-0 

70*0 

Congo 

68-0 

250 

48-0 

60*0 

ft          >»  tt 

Pontianak 

66-0 

sol. 

46*0 

63-0 

>>          >>  >» 

Sierra  Leone 

71-0 

62*0 

48-0 

57'o 

)>          >t  tt 

Manila,  hard  ,, 

73*o 

56-0 

58-5 

64-0 

,, 

soft  ,, 

64-0 

sol. 

287 

58-0 

>t          ft  tt 

Brazilian  ,, 

48-0 

38-0 

30-0 

40*5 

ft          ft  It 

Dammar 

sol. 

29'0 

4-0 

sol. 

Soluble. 

Mastic 

sol. 

36-0 

sol. 

tt 

Almost  insoluble. 

Sandarac 

74'o 

sol. 

67-0 

Partly  „ 

Shellac 

insol. 

sol. 

insol.  1 

almost 
insoluble 

) ,.  ,. 

Rosin 

sol. 

sol. 

sol. 

Partially  soluble. 

Elemi 

Soluble. 

Benzoin 

part  sol. 

part  sol. 

insol. 

Part  soluble. 

Madagascar  copal  . . 

6o*o 

74-0 

65-0 

78-0 

Insoluble. 

,,  fused 

4-0 

92*0 

52-0 

1*5 

Partly  soluble. 

after  naphtha- 

lene treatment 

52-0 

75-0 

20*0 

40*0 

tt  It 

The  ash  content  ought  to  be  very  low.  A  West  African 
copal  may  contain  0-2*2  per  cent.,  depending  on  the  grading. 

The  loss  in  weight  on  running  is  given  as  more  or 
less  25  per  cent.,  but  it  cannot  be  relied  on,  because  it  is 
entirely  dependent  on  the  process  employed.  Some  state 
that  the  desired  conditions  of  incorporation  with  linseed  oil 
can  be  effected  without  loss  in  weight  (H.  Terrisse  and 
Indestructible  Paint  Co.,  /.  5.  C.  7.,  1904,  2j,  582,  and 
1908,  ^7,  457).  The  process  is  essentially  a  depolymeriza- 
tion,  but  the  complexity  of  the  gums  must  admit  of  partial 
decomposition  of  component  acids,  with  the  liberation  of 
liquid  hydrocarbons  (copal  oil)  and  the  formation  of  acidic 
anhydrides  and  lactones. 

Spirit  Varnish  Resins 

The  second  class  of  resins  includes  dammar,  mastic,  and 
sandarac. 


88    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Dammar. — This  resin  is  a  typical  exudation  horn  Dammar 
orientalis,  indigenous  to  Dutch  East  Indies  and  British 
Malaya.  It  conies  into  the  market  in  the  form  of  nodules, 
clear  and  pale  in  colour.  Dammar  is  decidedly  softer  than 
the  majority  of  the  copals,  but  harder  than  rosin.  It  dries 
with  a  tack,''  and  as  a  spirit  varnish  it  gives  a  friable  coat 
which  can  be  easily  rubbed  up  as  a  powder.  Black  dammar 
is  an  Indian  resin. 

Mastic. — This  resin  is  the  most  important  European  form 
known  with  the  exception  of  colophonium.  It  occurs  in 
the  Mediterranean  littoral,  being  obtained  from  Pistachia 
lentiscus.  Much  comes  from  Greece  either  as  cake,  large  or 
small  mastic.  The  Pistachia  has  a  marked  wound  response, 
so  that  under  good  conditions  a  tree  may  yield  8-1  o  lbs.  per 
annum.  Mastic  is  used  as  picture  varnish,  and  with  boiled 
linseed  oil  forms    megilp,''  the  well-known  artists'  medium. 

Sandarac  is  a  North  African  resin  exudation  from  the 
Alerce  tree,  indigenous  to  North  Africa.  It  is  a  compara- 
tively hard  resin  and  is  used  to  impart  this  property  to 
mixtures  with  other  resins.  It  is  an  ingredient  of  negative 
varnishes,  label  varnishes,  and  bookbinders'  varnishes.  It 
is  soluble  in  methylated  spirit  and  in  ether,  but  only  partially 
soluble  in  turpentine,  petroleum,  and  benzol. 

Lac. — ^The  most  important  spirit  soluble  resin  is  Lac, 
considered  by  most  investigators  to  be  the  secretion  of  the 
lac  insect  {Tachardia  lacca)  which  is  found  on  a  number  of 
species  of  Indian  trees,  e,g,  acacia,  ficus,  mimosa,  etc. 
The  larvae  of  the  insect  puncture  the  bark  and  feed  on  the 
sap.  The  lac  excreted  gradually  imbeds  the  insects,  but 
respiration  is  maintained  through  passages  which  contain 
wax  penetrating  the  resin.  The  females,  after  fertilization, 
secrete  a  red  fluid,  the  lac  dye,  and  die  on  the  appearance 
of  new  larvae.  The  insects  do  not  move  from  that  part  of 
the  tree  on  which  they  first  swarm.  The  production  of  the 
lac  continues  until  the  tree  dies.  Recently  artificial  propaga- 
tion of  the  insect  has  been  resorted  to,  so  as  to  be  inde- 
pendent of  casual  distribution  by  birds  or  other  insects.  The 
province  of  Bengal  is  the  most  important  source  of  lac, 


RESINS  AND  PITCHES 


89 


but  it  is  produced  in  Ceylon,  Burmah,  Siam,  China,  and  the 
Malay  Archipelago. 

I^ac  has  several  forms  in  which  it  comes  on  to  the  market. 

(i)  Stick  lac,  (2)  Seed  lac,  (3)  Shellac,  (4)  Button  lac, 
(5)  Garnet  lac. 

(1)  Stick  lac  is  the  crude  product  direct  from  the  trees  in 
the  form  of  short  pieces  of  twigs  encrusted  with  lac. 

(2)  Seed  lac  is  obtained  by  breaking  the  twigs  and  remov- 
ing the  wood  followed  by  treatment  of  the  broken  lac  with 
warm  water  which  extracts  the  lac  dye :  this  may  be 
recovered  from  the  solution  by  evaporation. 

(3)  Shellac  is  obtained  from  seed  lac  by  fusion  and 
straining  the  melted  lac  through  cloth  bags.  The  molten 
lac  is  spread  over  the  surface  of  a  metal  or  porcelain  cylinder 
and  detached  with  a  knife  when  solid.  Orange  shellac  is 
sold  in  the  form  of  flakes  and  is  the  best  quality. 

(4)  Button  lac  is  shellac  in  large  round  fiat  pieces  of  a 
dark  ruby  colour. 

(5)  Garnet  lac  is  a  variety  which  has  been  deprived  to  a 
very  large  extent  of  wax and  appears  in  the  form  of 
thick  flat  pieces. 

The  native  methods  of  treatment  are  very  crude  and 
have  been  replaced  by  modern  methods  in  several  of  the 
Indian  factories.  The  uses  of  shellac  are  given  in  the  chapter 
on  spirit  varnishes. 

Stick  lac  contains  66  per  cent,  resin,  6  per  cent,  wax, 
6  per  cent,  gluten,  and  11  per  cent,  colouring  matter.  I^ac 
is  soluble  in  methylated  spirit,  giving  a  turbid  solution  due 
to  insoluble  wax.  It  is  insoluble  in  petroleum  spirit  but 
partially  soluble  in  ether,  chloroform,  and  turpentine  {vide 
Constants  and  Solubilities,  p.  87). 

Shellac  always  contains  rosin ;  which  is  added  in  the  manu- 
facture to  lower  the  fusion  temperature  of  the  lac  ;  orange 
shellac  usually  contains  1-2  per  cent.,  garnet  and  button 
lac  from  10-20  per  cent.  Increasing  amounts  of  rosin  render 
the  shellac  coating  friable  and  powdery.  Comparison  of 
the  iodine  values  of  shellac  and  rosin  (see  Constants)  indicates 
a  method  for  estimation  of  the  added  rosin. 


90    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 

Shellac,  when  applied  to  a  warm  metal  surface,  gives  a 
hard  lustrous  coat  which,  indoors,  is  a  very  valuable  pro- 
tective coating.  In  a  damp  atmosphere  it  disintegrates  and 
is  useless  for  outdoor  work.  The  film  becomes  rough  and 
powdery  and  does  not  regain  its  form  on  drying.  As  a 
hard  priming  undercoating  on  wood  under  varnish  it  is 
satisfactory,  especially  if  the  woodwork  contains  knots,  but 
it  is  stated  to  be  inadvisable  to  prime  knots  with  shellac 
previous  to  the  application  of  paint  (Gardner,  Paint 
Researches,''  1917,  p.  347).  Its  electric  insulating  properties 
are  high.  Shellac  is  soluble  in  alkalies,  alkaline  carbonates, 
and  borax.  This  property  is  utilized  for  the  preparation 
of  shellac  water  varnishes  and  to  produce  a  bleached  shellac 
soluble  or  partially  soluble  in  alcohol. 

Weak  alkaline  solvents  remove  the  colouring  matter  from 
the  crude  shellac.  It  is  remelted  and  pulled  out  in  warm 
water.  To  bleach  the  shellac  it  may  be  dissolved  in  alkalies 
and  chlorine  passed  in.  The  precipitated  lac  is  collected, 
melted  under  water,  and,  when  soft,  is  pulled  so  as  to  give 
it  a  fibrous  satinlike  appearance.  The  bleached  shellac 
has  to  be  kept  under  water  to  prevent  it  losing  its  solubility 
in  spirit,  and  even  under  these  conditions  it  gradually 
deteriorates.  The  spirit  soluble  form  is  a  component  of 
the  so-called  colourless  lacquers.  Bleached  shellac  is  rarely 
completely  soluble  in  methylated  spirit.  A  lac  water  varnish 
is  a  water  solution  of  shellac  or  bleached  shellac  in  borax  ; 
it  makes  a  good  waterproof  paper  varnish. 

Turpentine,  Rosin,  and  Rosin  Oil. — ^There  is  frequent 
misunderstanding  in  the  nomenclature  of  the  products  of 
the  resinous  exudations  of  the  varieties  of  Pinus.  Turpen- 
tine was  the  name  given  originally  to  the  exudations  which 
are  semifluid  and  in  their  raw  state  appear  in  the  market 
under  such  names  as  Venice  Turpentine  or  Strasburg  Turpen- 
tine. On  distillation  of  the  oleoresin,  turpentine  oil  spirits 
of  turpentine ")  is  obtained.  The  non-volatile  product 
is  rosin  (colophony,  colophonium) ,  which  on  dry  distillation 
furnishes  a  volatile  rosin  oil  and  a  residue  of  rosin  pitch. 
The  turpentine  used  in  Europe  is  practically  all  obtained 


RESINS  AND  PITCHES 


91 


from  the  United  States,  France,  and  Russia,  with  smaller 
quantities  from  Spain,  Portugal,  Algeria,  and  Greece.  Russia 
is  the  only  country  which  can  increase  its  output  largely, 
although  Russian  Turps  is  less  popular  than  the  American 
variety  for  reasons  stated  below.  Central  America  is  as 
yet  an  undeveloped  area  (Mexico  and  British  Honduras). 
Within  the  British  Empire,  Honduras  and  India  possess 
pines  which  yield  a  flowing  oleoresin,  but  in  India  the 
industry  has  been  developed  only  for  the  home  market. 
The  yield  of  crude  resin  (gum  thus)  varies  with  the  species 
of  Pinus.  In  America  incisions  are  made  in  the  trees  in 
winter  and  in  March  the  sap  begins  to  flow.  The  resin  is 
collected  in  suitable  boxes  attached  to  the  trees.  After 
four  years'  tapping  the  trees  are  felled  and  used  for  lumber. 
The  system  has  to  be  as  carefully  controlled  as  in  the  case 
of  rubber ;  if  the  tapping  is  too  drastic  the  tree  dies  or  the 
yield  falls  off,  moreover,  replanting  of  the  trees  must  be 
systematically  maintained  (Tschirch,  ''Die  Harze  u.  die 
Harzbehalter '').  The  most  important  American  varieties 
of  turpentine-yielding  pines  are  :  Pinus  australis  (Georgia 
pine)  and  Pinus  tceda  (the  loblolly  pine).  It  must  be 
pointed  out  that  resin  is  found  in  all  varieties  of  pine,  but 
only  a  few  give  a  flow  of  sap  after  incision  of  the  bark.  Five 
hundred  gallons  of  crude  turpentine  oleo  resin  yield  125 
gallons  of  turpentine  and  the  residue  is  rosin.  The  Central 
American  pine  is  Pinus  Cubensis,  The  French  variety  is  Pinus 
maritima  and  in  that  country  the  turpentine  industry  is  more 
carefully  regulated  than  in  America.  In  the  neighbourhood 
of  Bordeaux,  in  the  Departments  of  lyCS  I^andes  and  Gironde, 
the  cultivation  in  the  shifting  sands  of  the  Gascon  dunes 
has  been  developed  since  the  end  of  the  eighteenth  century.* 

Russian  turpentine  is  obtained  from  Pinus  sylvestris 
by  methods  which  are  wasteful  and  primitive  in  comparison 
with  those  adopted  in  America  and  France. 

In  Russia  and  Sweden  pine  stumps  are  placed  in  trenches 
dug  in  the  ground  having  iron-sheeted  bottoms  under  which 

*  For  a  comprehensive  account  of  the  French  turpentine  industry 
cf.  Joly,  Proc.  Oil  and  Colour  Chemists'  Assoc,  1920,  75-,  149. 


92    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


fires  are  started  to  distil  the  turps  from  the  stumps.  In 
India  there  are  large  tracts  of  pine  forest  in  the  Punjab,  the 
United  Provinces  of  Agra  and  Oudh.  The  production  of 
turpentine  in  191 1  in  the  United  Provinces  amounted  to 
27,000  gallons,  all  consumed  in  the  country.  Unfortunately 
it  is  slower  drying  than  the  American  variety,  containing  less 
volatile  terpenes  (F.  M.  Perkin,  Paint  and  Varnish  Soc,  1913). 

It  is  evident  that  unless  the  cultivation  of  the  tree  and 
the  extraction  of  the  resin  is  carried  out  in  an  economical 
manner  shortage  will  occur  and  prices  will  rise.  There 
are  many  resin-bearing  species  of  Pinus,  e.g.  Pinus  palustris 
(long-leaved  pine)  and  the  Douglas  fir  (British  Columbia), 
which  do  not  give  any  yield  on  tapping,  but  the  resin  may 
be  extracted  by  destructive  distillation  or  by  steam  distilla- 
tion. Another  source  is  the  by-product  in  the  wood  pulp 
industry,  especially  in  Scandinavia.  Wood  turpentine  is 
marked  by  a  peculiar  wood  essence-like  smell  and  apart 
from  this  possesses  the  main  properties  of  turpentine,  but 
is  always  coloured.  On  an  average  i|  gallons  of  turpentine 
are  obtained  from  100  cubic  feet  of  wood. 

The  oleoresins  from  the  genus  Pinus  may  be  used  as  such 
or  distilled  to  give  turpentine  and  rosin  (colophonium) . 

Canada  balsam  is  the  oleoresin  from  Abies  {Pinus) 
Canadensis;  Strasburg  turpentine  from  Abies  pectinata 
(silver  fir)  ;  Venice  turpentine  from  Pinus  larix  (larch) . 

According  to  Tschirch  {loc.  cit.),  Canada  balsam  (atypical 
oleoresin)  contains  :  canadinic  acid,  13  per  cent.  ;  canado- 
linic  acids,  48-50  per  cent.  ;  canadoresene,  7  per  cent. ; 
essential  oil  (turpentine),  23-24  per  cent. 

The  flowing  oleoresin  from  the  pines  of  France  and 
America,  etc.,  is  exuded  as  a  transparent  liquid  which 
becomes  viscid  and  turbid  in  contact  with  air.  Generally 
it  has  the  consistency  of  honey,  and  is  coloured  and  turbid. 
The  oleoresin  from  Pinus  maritima  contains  18  per  cent. 
tui*pentine,  70  per  cent,  rosin,  10  per  cent,  water,  and  2 
per  cent,  impurities.  The  yield  of  oleoresin  is  variable 
with  the  species ;  Tschirch  gives  20  per  cent.  (French  and 
American),  Schkateloff  gives  13-20  per  cent,  for  Russian  pines. 


RESINS  AND  PITCHES 


93 


To  obtain  turpentine  from  the  oleoresin  it  is  distilled  in  a 
current  of  steam  under  conditions  which  vary  locally  {vide 
F.  M.  Perkin,  loc.  cit.).  Wood  turpentine  oil  is  obtained  by 
the  destructive  distillation  of  tree  stumps  and  consequently 
many  additional  products  are  obtained,  e.g.  wood  alcohol, 
acetic  acid,  creosote  heavy  oils,  charcoal,  but  no  rosin.  In 
British  Columbia  the  pine  wood  is  distilled  in  brick  retorts 
electrically  heated  and  the  temperature  controlled  so  that 
it  does  not  rise  above  205°  C.  The  products  are  turpentine, 
rosin,  tar  oil,  tar,  and  charcoal  [lyivache  and  Mcintosh, 
Manufacture  of  Varnishes,''  Vol.  3,  2nd  edition]. 

Turpentine  is  a  mixture  of  terpene  hydrocarbons  and  its 
composition  varies  with  the  source.  In  America  turpentine 
dextro-pinene,  CioHig,  occurs ;  French  turpentine  contains 
terebenthene  (laevo-rotatory) ,  CioHig ;  Russian  turpentine 
contains  sylvestrene,  CioHig,  which  is  dextro-rotatory. 

Wallach  and  his  pupils  have  made  a  close  study  of  the 
natural  terpenes,  and  of  late  years  a  number  of  terpene 
hydrocarbons  have  been  synthesized  by  Perkin,  so  that  the 
structural  formulae  are  fairly  well  established.  In  practice  it 
is  rarely  necessary  to  isolate  any  of  the  characteristic  deriva- 
tives of  the  terpenes  and  the  standards  given  below  are 
adequate  : — 

%  fraction 

Flash  point  boiling  below  %  fraction  boiling 
S.G.  i5°C.         [Abel].  i6o°  C.  below  170°  C. 

American  turpentine    o*862-o-87    86-88°  F.    72-78%  95-97% 

French  turpentine  . .       0*864  »>  »  »» 

Russian  turpentine         o-86i         93°  F.        8  o/^     g5%  below  1 70^  C. 

(below  i7o«'C.— 35% 
0-859  »  —  -  i75^C.-8oo/o 

(abovei8oX.-20% 

It  must  be  remembered  that  pinene  undergoes  decomposi- 
tion above  250  °  C.  and  gives  resinous  products,  so  that  care 
has  to  be  taken  not  to  exceed  that  temperature  during  the 
dry  distillation  of  wood. 

The  terpenes  are  ring  compounds  containing  unsaturated 
linkages  and  asymmetric  carbon  atoms,  so  that  the  variety 
of  their  derivatives  is  great  and  their  activity  is  very 
marked. 


American  wood  tur- 
pentine   . . 


94    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Constants  of  Terpenes. 

B.P.  S.G.  [a]jy  Hydrochloride.  Nitrosochloride. 

Pinene       ..  155-156°  C.   0-857/20°  j  T«:n  C,„H,eHCl,  m.p.  125°  C.   Ci„HieNOCl  m.p. 

Sylvestrene. .       177°          0-851  +66-3  \^io^u^'^^^  •  ^iq^i^^HX  CjoHigNOCl.m.p. 

^  m.p.  i35°C.)             io6°-7^  C. 

Limonene  ..       176°          0*846  +106*8   CjoHigBr^  m.p.  104°  C.  — 

Phellandrene    iyo°-2°  C.       —  —       nitrite,  ra.p.  i03°-4°  C.  — 

The  properties  of  the  last  two  are  mentioned  because  of 
their  connection  with  the  products  of  the  running  of 
copals. 

The  important  properties  of  turpentine  from  a  practical 
standpoint  are  :  (i)  solvent  properties  for  oils  and  resins  ; 
(2)  drying  properties  and  action  as  a  catalytic  oxygen  carrier. 
Turpentine  is  a  good  solvent  for  sweated  copals,  rosin  oils, 
for  metallic  salts  of  drying  oils  and  metallic  resinates,  e.g. 
manganese  linoleate  and  manganese  resinate. 

American  and  French  turpentine  evaporate  easily  at 
the  ordinary  temperature,  leaving  a  slight  greasy  residue. 
Russian  turpentine  leaves  a  much  larger  greasy  residue, 
similarly  wood  turpentine,  so  that  these  two  varieties  are 
not  considered  to  dry  hard.  Turpentine  absorbs  oxygen 
readily  from  the  air  forming  a  peroxide  :  when  pinene  is 
exposed  to  the  air  and  light  sobrerol,  C10H16O2,  is  formed 
(Armstrong).  No  doubt  during  the  evaporation  of  turpen- 
tine greasy  oxidation  products  are  formed  which  may  bind 
any  dissolved  resin ;  generally  speaking  any  catalytic 
oxidizing  powers  may  be  considered  small  compared  with 
those  of  drying  oils  (cf .  Turpentine  Substitutes) . 

Turpentine  Substitutes 
With  the  steadily  rising  price  of  turpentine  and  the 
prospect  of  world  shortage,  unless  the  cultivation 
of  the  pine  forests  is  methodically  organized,  it  is  only 
natural  that  substitutes  for  turpentine  will  be  welcomed. 
The  requirements  are  solvent  power  equal  to  that  of  turpen- 
tine and  air-drying  power  as  rapid,  leaving  little  or  no  volatile 
residue.  The  flash  point  must  be  above  73°  F.  to  conform 
with  transport  requirements.  The  smell  must  be  pleasant 
and  resemble  that  of  turpentine  as  much  as  possible.  The 


RESINS  AND  PITCHES 


95 


basis  of  turpentine  substitutes  nowadays  is  a  petroleum 
blended  with  turpentine  in  varying  amounts.  If  a  petroleum 
can  fulfil  the  above  requirements  the  use  of  turpentine  is 
unnecessary.  A  short  reference  must  be  made  to  several 
factors  which  are  of  considerable  importance. 

The  petroleums  are  generally  not  such  good  solvents  for 
gum-oil  mixings  or  for  metallic  driers,  nor  have  they  the  same 
viscosity  and  flow  as  turpentine.  From  the  experiences 
of  the  1906  Test  Fence  of  the  North  Dakota  Experimental 
Station  (Ladd  and  Washburn,  Bulletin,  1915,  /,  73),  the 
substitution  of  petroleum  for  turpentine  does  not  give  the 
same  results  as  where  turpentine  in  moderate  amount  is 
used.  It  is  probable  that  the  slight  greasy  residue  of 
oxidized  turpentine  facilitates  the  retention  of  the  resin  in 
solution  (raw  resins  are  more  soluble  in  oxidized  turpentine 
than  in  turpentine) .  Friend  [loc,  cit.)  finds  that  for  paints  on 
iron  turpentine  and  petroleum  media  are  equal.  The  petro- 
leums offered  are  so  varied  in  composition  that  for  comparison 
only  volatility  tests  can  be  carried  out ;  generally  the  presence 
of  aromatic  hydrocarbons  tends  to  improve  their  solvent 
power.  There  would  appear  to  be  no  definite  evidence  that 
turpentine  assists  the  catalytic  oxidation  of  the  gum-oil  drier 
mixing.  It  was  supposed  that  any  kind  of  petroleum  would 
cause  ''bloom  in  varnishes,  but  this  is  incorrect.  For  the 
estimation  of  petroleum  in  turpentine  cf.  lyunge  and  Keane, 
''Technical  Methods  of  Chemical  Analysis/'  Part  i.  Vol.  3, 
also  Chem.  Zeit.  1918,  4.2^  349-351. 

Colophony  (Rosin). — In  the  distillation  of  turpentine 
oleoresin  in  a  current  of  steam,  after  the  turpentine 
has  come  over,  rosin  is  left  in  the  stills.  The  purification 
of  French  rosin  includes  (i)  drying  to  expel  water,  which 
would  otherwise  leave  the  rosin  opaque,  (2)  filtration  of  the 
fused  rosin,  (3)  moulding  whereby  the  filtered  molten  rosin 
is  run  into  suitable  moulds,  (4)  bleaching  by  exposure  of  the 
moulded  rosin  to  the  sun's  rays  (lyivache  and  Mcintosh,  loc, 
cit).  Rosins  are  graded  according  to  paleness;  ''window 
glass  ''  rosin  is  clear  and  of  a  pale  amber  colour  ;  common 
rosin  is  clear  but  darker  ;  black  rosin  is  opaque  and  very  dark. 


96    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Rosin  contains  abietic  acid  C20H40O2,  with  its  isomers 
pinic  acid  and  sylvic  acid  (Paul,  Seif,  fabr.,  1916,  j6,  545  ; 
Z.  angew.  Chemie,  1915,  2S,  415). 

American  rosin  contains  90  per  cent,  pinic  acid  (m.p. 
70°  C.)  and  sylvic  acid  (m.p.  122°  C).  Pinic  acid  has  been 
isolated  in  three  varieties,  a,  /3,  y,  with  different  melting 
points,  y-pinic,  sylvic,  and  y-abietic  acids  are  soluble  in 
petroleum  ether  while  a  and  j3-pinic  acids  are  insoluble. 
Abietic  acid  from  Spanish  rosin  is  identical  with  the  acid 
obtained  from  American  colophony  (Blanes,  /.  Chem,  Soc, 
1915,  loS,  1493). 

The  formula  C20H30O2  proposed  by  Fahrion  {Chem. 

C.CH3  C.COOH 

HC    CH— C  CH 

I  I 

H2C    CH — C  CH2 

\/  \/ 

C3H7.HC  CH.C3H7 

Revue,  1913)  was  put  forward  by  Bischoff  and  Nastvogel  in 
1880.  As  in  the  case  of  the  copal  resins  much  research  work 
is  yet  needed  in  elucidating  the  composition  and  formula  of 
abietic  acid  and  its  isomers. 

Colophony  (colophonium)  contains  rescues  which  in 
the  case  of  copal  rescues  may  be  regarded  as  oxypolyter- 
penes  formed  from  terpenes  by  simultaneous  oxidation  and 
polymerization.  In  spite  of  uncertainty  of  its  formula  sufficient 
is  known  to  connect  its  probable  structural  formula  with  its 
most  important  practical  uses.  It  is  an  acid  resin,  soluble 
in  most  organic  solvents,  giving  a  lustrous  film  of  low  dura- 
bility and  hardness  ;  it  is  an  unsaturated  body,  able  to  absorb 
oxygen  and  reacting  with  metals,  e,g,  lead,  manganese,  and 
cobalt,  in  the  same  way  as  the  drying  oil  acids,  i.e,  as  an 
oxidizing  catalyst.  In  this  connection  the  resinates  of  lead 
and  manganese  and  cobalt  are  largely  used  ;  they  are  soluble 
in  turpentine  and  to  a  slightly  less  degree  in  the  turpentine 
substitutes.  The  tackiness  of  rosin  films  can  be  corrected  by 


RESINS  AND  PITCHES 


97 


addition  of  lime  or  zinc  oxide  whereby  the  film  is  hardened, 
but  it  is  prone  to  hydrolysis  by  water. 

Rosin  esters  can  be  obtained  by  condensation  of  alcohols 
under  various  conditions,  e.g,  gtycerol,  resorcin,  naphthol. 
The  ester  gums  are  pale  in  colour,  unacted  on  by  water  and 
can  be  easily  incorporated  with  oil,  thereby  improving  the 
water-resisting  properties  of  the  film.  The  properties  of 
ester  gums  have  been  investigated  by  Ellis  and  Rabinovitz 
(/.  Ind.  Eng.  Chem,,  1916,  <?,  406),  with  special  consideration 
for  preventing  livering  "  of  the  vehicle  in  the  presence 
of  pigments.  This  paper  is  of  interest  in  giving  a  summary 
of  the  published  work  to  date.  Rosicki  [Farb,  Zeit,,  1913, 
1 1 94)  considers  that  the  reduction  in  the  acidity  of  the 
resins  is  a  necessary  condition,  whereas  Muehle  i^hid.,  1913, 
119, 1944  ;  2058  and  2178)  is  of  opinion  that  the  coagulation 
livering is  due  to  association  of  copal  particles  rather  than 
to  the  formation  of  insoluble  salts,  because  he  was  unable 
to  produce  varnishes  from  Congo  and  Manila  copals  which 
would  stand  the  addition  of  pigments  either  after  esterifica- 
tion  or  reduction  in  the  acid  value.  Meguele  (ihid.y  1913, 
2230)  considers  there  is  truth  in  both  views  and  the  verdict  of 
practical  experience  will  support  him. 

Rosin  Oil. — When  rosin  is  heated  in  a  closed  vessel  it 
undergoes  destructive  distillation  ;  the  heating  may  be  by 
direct  fire  with  or  without  introduction  of  superheated  steam. 
The  conditions  of  distillation  modify  the  proportions  of  the 
products.    The  products  consist  of  : — 


Gas 

Acid  water 
Rosin  spirit 
Rosin  oil  . . 
Pitch 


Fire  heat  and  super- 
Dry  distillation.  heated  steam. 

.    5-4%  - 

•    2-5  %  (containing  10  %  acetic  acid)  — 

.    3*1  15-0 

.  85-1  64% 
.  3'9 


The  rapidity  of  the  distillation  has  an  effect  on  the  propor- 
tion of  the  products.  Hard  rosin  oil  is  produced  when  the 
distillation  is  conducted  rapidly  and  during  the  first  stages 
of  distillation,  whilst  soft  rosin  oil  is  produced  when  the 
process  is  conducted  slowly  and  during  the  middle  period 
of  the  distillation.  The  gas  is  of  value  for  illuminating 
s.  7 


98    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


purposes,  and  from  the  acid  water  calcium  acetate  is  pre- 
pared. The  properties  of  rosin  pitch  will  be  referred  to  under 
''Pitches."  In  France  the  addition  of  lime  to  the  rosin 
causes  an  alteration  in  the  composition  of  the  products.  For 
the  paint  and  varnish  industry  the  demand  for  rosin  oil  is 
insignificant  compared  with  its  utilization  as  a  lubricant. 
The  better  qualities  of  rosin  oil  have  no  drying  properties 
and  the  crude  varieties  are  feeble  in  that  respect. 

Rosin  spirit  is  said  to  contain  a  fluid  hydrocarbon 
C7H12,  b.p.  103-4''  C.,  and  hexylene  and  amylene  occur 
in  light  rosin  spirit.  Rosin  spirit  may  be  present  in  turps 
substitutes,  but  if  the  latter  contains  rosin  oil  it  will  dry 
greasy  and  will  not  distil  over  below  250°  C. 

Chinese  and  Japanese  Lacquer. — ^There  are  two 
genera  of  plants  belonging  to  the  natural  order  Anacardi- 
acecB  containing  species  which,  on  being  tapped,  yield  a 
sap  largely  used  in  the  East  as  natural  varnish  or  lacquer  ; 
these  are  Rhus,  which  yields  Japanese  lacquer,  and  Melanor- 
rhea  which  produces  the  black  varnish  of  Burma.  The  Japan 
lacquer  is  characterized  by  : — (i)  hardness,  which  increases 
with  age,  (2)  lustre,  which  is  retained  under  varying  atmo- 
spheric influences,  and  (3)  resistance  to  the  usual  agencies 
which  attack  varnishes  (Bulletin  of  the  Imperial  Institute, 
1910,  8y  32).  The  tree  which  yields  Japanese  and  Chinese 
lacquer  is  Rhus  vernicifera.  It  is  found  in  woods  at  an 
elevation  of  4000  feet  and  cultivated  along  the  margins 
of  fields  or  valley  bottoms.  The  tapping  of  the  tree  for 
sap  resembles  that  of  the  Pinus  for  turpentine  oleoreski. 
Ivarge  quantities  of  the  varnish  pass  annually  through  the 
Port  of  Hankow.  In  1908,  2,479,702  lbs.  were  exported, 
the  bulk  of  which  went  to  Japan.  The  raw  varnish,  which  is 
frequently  adulterated  with  Tung  oil,  is  known  in  Japan  as 
Ki-urushi.  When  first  collected  the  sap  is  of  grey-brown 
colour  of  viscid  consistency,  turning  black  on  exposure  to 
the  air  and  becoming  coated  with  a  thick  tough  skin.  The 
only  method  of  thinning  the  lacquer  known  to  the  Japanese 
is  by  adding  camphor.  The  peculiarity  of  the  substance 
is  that  it  hardens  only  in  a  moist  atmosphere  and  remains 


RESINS  AND  PITCHES 


99 


in  a  tacky  condition  if  exposed  to  sunlight  and  heat.  Its 
application  in  China  is  performed  only  in  wet  weather  or  in 
a  damp  atmosphere.  Oxygen  to  575  per  cent,  by  weight 
is  absorbed  in  drying  at  the  ordinary  temperature :  whether 
the  catalyst  is  an  ordinary  ferment  (laccase)  or  the  activity 
is  due  to  the  presence  of  manganese  with  a  proteid-like 
substance  is  undecided.  For  a  concise  description  of  the 
manufacfure  of  Chinese  and  Japanese  lacquer  reference 
may  be  made  to  Bulletin  of  the  Imperial  Institute  {loc.  cit). 

The  Burmese  black  varnish  or  lacquer  (thitsi)  is  an 
oleoresin  obtained  from  the  black  varnish  tree  Melanorrhea 
usitata.  The  trees  are  tapped,  and  in  general  properties 
the  lacquer  is  similar  to  that  obtained  from  Rhus,  although 
slower  in  drying.       Drugs,  Oils,  and  Paints,''  1917,  J2,  413.) 

The  application  of  the  lacquer  is  said  to  be  dangerous 
to  western  workers  owing  to  the  peculiar  poisonous  properties 
of  the  resin.*  Majima  and  Tahara  {Ber.,  1915,  ^8,  1593) 
have  synthesized  hydrourushiol  dimethyl  ether  obtained 
from  urushiol/'  the  chief  constituent  of  Japan  lac.  From 
their  investigations  Japan  lac  is  an  aromatic  derivative 
with  a  long  side  chain,  since  hydrourushiol  dimethyl  ester 
has  been  shown  to  have  the  formula, 

(CH30)2C6H3(CH2)uCH3. 

For  durability  Japan  lacquer  far  surpasses  any  other 
resin  known. 

Synthetic  Resins. — When  phenols  and  formaldehyde 
(formalin)  are  heated  in  the  presence  of  condensing  agents 
substances  are  obtained  which  in  appearance  and  properties 
resemble  natural  resins,  especially  Indian  lac.  The  con- 
densing agent  may  be  an  acid,  an  alkali,  or  ammonia.  The 
investigation  of  these  substances  has  been  developed  b}^ 
Baekeland  and  his  collaborators.  The  chief  representative 
of  the  class  of  synthetic  resins  is  Bakelite,  which  is  manu- 
factured in  a  number  of  forms,  each  with  important 
properties.  An  account  of  Bakelite  and  its  applications 
is  given  by  H.  I^ebach  (/.  5.  C.       1913,  j2,  559).  The 

*  Recent  experience  has  shown,  however,  that  the  poisonous  properties 
have  been  much  exaggerated. 


100    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


so-called  Bakelite  varnishes  are  solutions  of  Bakelite  A 
in  methylated  spirit.  The  varnishes  when  stoved  give 
hard  insoluble  and  infusible  coatings  of  high  chemical  and 
mechanical  resistance.  Stoved  Bakelite  lacquer  films  resist 
the  action  of  methylated  spirit,  ammonia,  and  salt  water, 
moreover  they  are  acid  proof.  Bakelite  is  used  for  the 
impregnation  of  coils,  armatures  for  magnetos,  arc  lamps, 
transformers,  etc.,  because  of  its  great  dielectric  ^strength. 
If  fluid  Bakelite  A  "  (Resole)  is  heated  for  several  hours 
at  a  temperature  of  140 ""-170°  C,  and  at  a  pressure  of  10 
atmospheres,  it  is  transformed  into  transparent,  insoluble, 
and  infusible  Resite.  Generally  a  filling  material  is  used, 
which  is  impregnated  with  the  fused  resole  forming  a  plastic 
mixture,  which  completely  fills  the  mould  in  the  press. 
At  a  temperature  of  ioo°-20o''  C.  the  resole  is  changed  to 
Bakelite  C (Resite).  Resite  is  non-hygroscopic  and  is 
the  most  resistant  of  all  plastics  ;  it  can  be  heated  to  300°  C. 
without  decomposition  and  at  higher  temperatures  it  chars 
but  does  not  ignite.  It  is  suitable  for  pen  and  pencil  holders, 
umbrella  handles,  cigar  holders,  etc. 

The  Resites  are  considered  by  Wohl  {Ber.,  1913,  ^5,  2046) 
to  be  polymerization  products  of  methylene  derivatives  of 
the  tautomeric  phenol 

CH  CH 
CH2.CH<^  ^C=0 
CH~CH 

Furfuraldehyde  can  replace  formaldehyde  in  these  resins 
(/.  5.  C.  /.,  1920,  577A) .  In  oil  varnishes  (Albert  and  Behrend, 
Eng.  Pat.,  15875/1914,  and  Ger.  Pat.,281939/1913)  they  have 
had  only  a  limited  success.  A  new  class  of  artificial  resins 
(cumarone  resins)  has  attracted  attention  lately.  These  resins 
are  polymerization  products  of  cumene  and  indene  (obtained 
from  crude  benzol  (b.p.  160 ""-180°  C.)  by  the  action  of  sul- 
phuric acid  (Bottler,  Kunstoffe,  1915,  5,  277,  and  Krumbhaar, 
Farb,  Zeit.,  1916,  21,  1086).  The  cumarone  varnishes  are 
said  to  be  tacky,  and  although  they  can  be  hardened  by 
the  addition  of  paraindene,  yet  their  durability  is  poor. 


RESINS  AND  PITCHES 


101 


Unaccountable  thickening  ensues  when  cumarone  varnishes 
are  mixed  with  certain  pigments. 

The  acrylic  acid  esters  prepared  from  glycerine,  lactic 
acid,  etc.,  when  exposed  to  sunlight  or  ultraviolet  light  poly- 
merize to  varnish^ike  elastic  masses.  They  are  soluble  in 
solvents  for  oils  and  are  stated  to  dry  rapidly  and  not  to 
be  readily  affected  by  exposure  or  by  chemical  agents 
(/.  5.  C.  /.,  I9i6,jj',  698). 

Accroides  Resin.  —  Xanthorrhea,  or  grass  tree 
[Juncacece],  is  confined  to  Australia  and  Tasmania.  It  is  a 
plant  with  a  short  thick  woody  stem,  terminated  by  a  tuft 
of  long  leaves  about  3  feet  with  cutting  edges.  The  three 
most  important  species  are  X,  hastilis  and  arborea  (red  gum 
accroides),  and  X.  australis  (yellow  gum  accroides).  From 
the  trunk  of  the  trees  on  incision  flows  a  resinous  substance 
giving  a  layer  2-4  cm.  thick  in  the  case  of  X.  australis. 
Sometimes  fragments  detach  themselves  spontaneously 
and  collect  at  the  foot  of  the  tree  where  they  are  found 
buried  and  semifossilized. 

Red  gum  accroides  approximatesDragon's  Blood  in  colour, 
with  a  shade  approaching  brown,  possessing  an  orange 
streak.  In  lustre,  however,  it  is  superior.  The  yellow 
variety  differs  only  in  colour  and  in  structure  from  the  red 
variety  (I^ivache  and  Mcintosh  {loc.  cit.),  Vol.  3,  and  Seelig- 
man  and  Zieke,  "Handbuch  der  I^ack  u.  Firnis  Industrie'' 
(1914)).  Both  varieties  are  soluble  in  alcohol  and  alkalies 
but  insoluble  in  petroleum  ether,  in  that  respect  resembling 
shellac.  Xanthorrhea  resins  seem  to  belong  to  the  same 
class  chemically  as  Peru  balsam,  storax,  and  benzoin.  During 
the  War  it  was  suggested  that  they  might  be  transformed 
into  picric  acid  by  comparatively  simple  nitration. 

Pitches 

Pitches. — ^The  mineral  pitches  comprise  bitumen  or 
asphaltum  (French,  Asphalte  ;  Spanish,  Asfalto  ;  German, 
Erdpech).  The  term  ''bitumen  "  is  considered  by  I^angton 
{Proc.  Oil  and  CoL  Chem,  Assoc,  1919)  to  be  generic, 
defining  a  class  of  substances  soluble  in  carbon  disulphide 


102    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


and  other  neutral  liquids  and  consisting  of  compounds  of 
carbon  and  hydrogen  associated  frequently  with  compounds 
of  oxygen,  sulphur,  and  nitrogen.  Asphalt  would  be 
regarded  as  mineral  matter  containing  bitumen  in  intimate 
association. 

The  artificial  pitches  or  residual  pitches  (Richardson, 
/.  Ind.  Eng.  Chem.,  1916,  ,  4)  are  essentially  the  products 
of  the  distillation  or  destructive  distillation  of  carbonaceous 
material  in  the  form  of  coal  (coal-tar  pitch),  petroleum 
(mineral  oil  pitch),  resinous  woods  (Stockholm  pitch,  rosin 
pitch),  vegetable  and  marine  oils  (stearine  pitch),  wool  fat 
(wool  grease  pitch),  and  bones  (bone  pitch). 

The  above  list  shows  their  great  variety  with  end- 
less complexity  of  chemical  composition,  yet  marked  by 
characteristic  differences  as  to  hardness,  blackness,  fusibility, 
and  brittleness. 

In  order  of  magnitude  of  production  coal-tar  pitch  is 
the  most  important. 

Coal-tar  Pitch  is  the  residue  remaining  in  the  stills 
after  the  first  distillation  of  coal-tar  and  amounts  to  two- 
thirds  of  the  weight  of  the  treated  tar  or  4  per  cent,  of  the 
weight  of  the  coal  carbonized.  The  lower  gravity  tars, 
produced  at  low  carbonization  temperatures,  are  rich  in 
open  chain  paraffinoid  bodies  and  give  a  lower  yield  of  pitch 
than  the  high  gravity  tars  containing  aromatic  hydrocarbons 
with  free  carbon.  The  softness  and  hardness  of  tar  pitch 
depend  on  the  conditions  of  distillation  :  a  harder  pitch 
being  produced  by  heating  the  tar  strongly  to  obtain  the 
maximum  yield  of  anthracene  distillate  and  the  residual 
hard  pitch  is  graded  by  addition  of  creosote  or  anthracene 
oil.  Stewart  {Trans.  London  and  Southern  District  Junior 
Gas  Association,  1911-12,  43)  states  that  at  high  temper- 
atures and  with  light  charges  a  yield  of  78  per  cent,  pitch  is 
obtained ;  continual  distillation  from  vertical  retorts  gives 
47  per  cent ,  whereas  moderate  heat  and  fairly  heavy 
charges  give  56  per  cent,  pitch  of  finer  quality  and  a  lower 
content  of  free  carbon.  The  specific  gravity  varies  from 
1-2  to  1-3.    Soft  coal-tar  pitch  softens  at  37  ""C.  and  melts 


RESINS  AND  PITCHES 


103 


about  60°  C.  Moderate  hard  pitch  softens  at  60°  C.  and 
melts  at  80  °C.:  the  hard  variety  softens  at  80°  C.  and 
melts  at  about  175  °C.  The  melting  point  is  taken  as  the 
point  at  which  the  pitch  becomes  soft  enough  to  flow.  I^ittle 
is  known  of  the  constituents  except  that  they  belong  to 
the  aromatic  series  (Marcusson,  Z.  angew.  Chem.,  1918,  ji, 

Carboa.       Hydrogen.       Oxygen.  Sulphur. 
Hard  coal-tar  pitch  •  •    75'3  ^'4  — 

Trinidad  asphalt  (Key ser)  . .    78*8  11*4  —  io*o 

For  the  varnish  maker  the  use  of  coal-tar  pitches  is 
limited  by  partial  solubility  in  petroleum  although  they  are 
freely  soluble  in  coal-tar  spirit. 

Petroleum  Pitches. — rival  to  the  coal-tar  pitches 
are  the  residues  left  in  the  refining  of  petroleum.  From 
all  the  refineries  in  America,  Galicia,  and  Rumania  grades  of 
pitch  are  obtained  suitable  as  road-binding  material,  asphalt, 
and  cements.  In  1917  the  United  States  produced  701,809 
short  tons  of  petroleum  pitches.  In  England  these  pitches 
are  unable  to  compete  with  the  cheaper  coal-tar  varieties 
for  ordinary  classes  of  work.  They  resist  the  action  of 
acids  and  alkalies  and  possess  good  lustre,  tenacity,  high 
electric  resistance,  and  adhesive  power  to  metal  surfaces. 
Some  oils  yield  a  softer  pitch  soluble  in  turpentine  or  petro- 
leum so  that  it  can  be  used  in  Japans  or  cycle  enamels. 

Bitumen  or  Asphaltum. — Bitumens  are  preferred  for 
varnish  purposes  because  of  their  hardness,  lustre,  and 
durability,  and  they  furnish  excellent  black  enamels.  Some 
of  the  commoner  varieties  are: — Grahamite  (Oklahoma), 
Gilsonite  (Utah),  Bermudez  (Venezuela),  Man jak  (Trinidad), 
in  addition  to  bitumens  from  Cuba,  Colombia,  and  Syria. 
Some  asphalts,  e.g.  Trinidad  and  Bermudez,  are  refined  before 
being  marketed.  The  natural  asphaltums  are  generally 
soluble  in  petroleum  except  Trinidad  pitch  (partially  soluble) 
and  ozokerite  pitch.  They  are  unacted  on  by  dilute  acids 
and  alkalies.  For  varnish  making  they  should  possess 
brilliant  lustre,  a  conchoidal  fracture,  and  not  lose  shape  in 
boiling  water.    They  should  be  practically  free  from  ash 


104    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


and  be  soluble  in  turpentine,  petroleum,  and  high  boiling- 
point  coal-tar  naphtha.  The  specific  gravity  varies  between 
I'l  and  1*2.  The  pitches  soften  at  60""  C.  and  melt  between 
100°  and  140°  C.  A  summary  of  the  literature  on  the 
subject  may  be  found  in  papers  by  lyangton  {loc.  cit)  and 
Donath  and  Strasser  {Chem.  Zeit,,  1893,  77,  1788).  An 
analysis  of  a  bitumen  shows  the  presence  of  carbon,  hydrogen, 
and  sulphur  in  approximately  the  following  amounts 
which  vary  with  the  source  :  carbon,  74  per  cent. ;  hydrogen, 
10  per  cent ;  sulphur,  16  per  cent. 

Stearine  Pitches. — Stearine  pitches  are  the  residues 
from  the  distillation  of  vegetable  and  mineral  oils  or  fatty 
acids.  They  are  of  value  for  blending  with  bitumens,  possess- 
ing high  lustre,  satisfactory^  hardness,  elasticity  and  solubility 
in  turpentine  and  in  petroleum.  The  pitches  from  drying 
and  semi-drying  oils  are  usually  less  soluble  in  the  above 
solvents.  Cotton-seed  oil  pitch  is  made  by  distilling  cotton- 
seed black  grease,  prepared  from  the  mucilage  obtained  in 
refining  crude  cotton-seed  oil. 

When  unrefined  cotton-seed  oil  is  distilled  the  pitch  is 
obtained  in  the  form  of  a  highly  elastic  spongy  mass  :  this 
elastic  cotton-seed  pitch  can  be  vulcanized  and  in  that 
form  is  largely  used  in  electric  cables,  but  the  coating  is 
apt  to  be  brittle  at  low  temperatures.  The  harder  varieties 
are  used  in  the  manufacture  of  waterproof  paper. 

Stearine  pitches  give  a  lustrous  coating  which  is  brown 
in  thin  layers  ;  this  defect  may  be  neutralized  by  incorpora- 
tion with  Bone  Pitch,  an  intensely  black  pitch  obtained  by 
the  distillation  of  bone  oil  (Dippel's  oil) ,  produced  when  bones 
are  dry  distilled.  Bone  oil  yields  about  23  per  cent,  of  hard 
pitch.  Alone,  bone  pitch  is  not  very  soluble  in  solvents,  but 
it  can  be  blended  with  more  soluble  varieties.  All  the 
stearine  pitches  require  sweating  before  incorporation  into 
black  varnishes  to  give  coatings  of  satisfactory  dryness. 

Wool  grease  pitch  is  softer  than  stearine  pitch  (Donath 
and  Margosches,  Chem.  Rev,,  1914,  1904),  and  is  used  as  a 
lubricant  rather  than  as  a  varnish  pitch. 

Stearine  pitches  can  also  be  obtained  from  whale  oil 


RESINS  AND  PITCHES 


105 


fatty  acids.  It  is  probable  that  the  constituents  of  stearine 
pitches  are  decomposition  products  of  fatty  acids  and  esters 
consisting  of  paraffinoid  hydrocarbons,  ketones,  polymeriza- 
tion products,  and  complex  esters  which  are  more  weather- 
resisting  than  the  original  oils.  The  softness,  elasticity,  and 
solubility  depend  on  the  source  of  the  material  and  on  the 
conditions  of  distillation. 

Ozokerite. — ^Among  the  mineral  pitches  Galician  ozo- 
kerite gives  on  distillation  a  hard  waxy  substance,  breaking 
with  a  rough  granular  fracture.  In  colour  it  is  dark  amber, 
softening  at  50  °C.  and  melting  between  85°  C.  and  100  "^C. 
Ozokerite  and  ceresin  (the  product  obtained  by  refining 
ozokerite)  are  employed  as  finishing  wax  and  in  electric 
cables.  Some  qualities  burnish  well  and  take  a  high  polish 
(Redwood,  /.  Soc.  Arts,  1886,  34 ;  Marcusson  and  Schulter, 
/.  S.  C.  /.,  1905,  26,  491). 

Stockholm  or  Swedish  pitch  and  Rosin  pitch  are 
typical  products  of  the  distillation  of  residues  of  wood  tar 
and  of  rosin  respectively. 

Stockholm  tar  when  distilled  gives  light  oils,  sp.  gr.  0*84- 
0*88,  and  about  70  per  cent,  yield  of  pitch.  The  pitch 
is  soft  with  a  jet-like  lustre,  but  of  a  brown  colour  in  thin 
films.  It  is  easily  soluble  in  alkalies  owing  to  the  presence 
of  phenolic  bodies,  e.g.  guiacol  and  cresol,  which  endow  it 
with  marked  preservative  and  antiseptic  properties  whereby 
it  is  largely  used  in  the  painting  of  ships.  It  enters  into  the 
composition  of  many  impermeable  cements  and  black  var- 
nishes. Prior  to  1914  the  annual  yield  from  Russian  sources 
was  estimated  at  55,300  tons. 

Rosin  pitch  is  the  residue  left  on  distillation  of  rosin 
(colophony)  and  amounts  to  about  15  per  cent,  of  the  rosin 
taken.  It  is  a  yellowish-brown  substance  with  a  rosin-like 
smell,  possessing  a  sticky  feel  and  crumbling  easily  on  slight 
pressure.  It  dissolves  in  nearly  all  the  solvents  for  colophony. 
Rosin  pitch  is  rarely  used  alone  in  varnishes  and  is  only 
occasionally  blended  with  low-grade  pitches. 

In  the  examination  of  pitches  the  determination  of  the 


io6    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


softening  and  melting  points  is  of  importance  as  well  as  the 
specific  gravity.  In  the  stearine  pitches  the  acidity  and 
saponification  values  are  useful,  but  in  very  hard  fat  pitches, 
obtained  by  pushing  the  distillation  to  its  utmost  limit, 
the  values  are  low.  A  comparison  of  the  acid  and  saponi- 
fication values  of  stearine  pitch  and  petroleum  pitch  is  shown 
in  the  subjoined  table  : — 

Acid  value.  Saponification  value. 

Stearine  pitch  .  .  0*2-2*9  >  ^'4  2*2-8*3 ;  4*3 
Petroleum  pitch   ,  .      0*1 ;  i*2  ;  0*3       1*3  ;  2*6 ;  1*7 

The  detection  of  wood-tar  pitch  is  facilitated  by  the  charac- 
teristic smell  of  creosote  on  heating  and  by  complete 
solubility  in  alcohol.  Coal-tar  pitch  contains  a  considerable 
amount  of  free  carbon. 

The  methods  of  identification  of  the  varieties  of  pitches 
are  on  the  whole  unsatisfactory.  A  scheme  has  been  put 
forward  by  Mansbridge  (/.  S.  C.  /.,  1918,  j/,  184)  which 
applies  to  single  pitches,  but  not  to  mixtures.  It  relies  on 
a  division  into  saponifiable  and  unsaponifiable  pitches  and 
the  behaviour  of  the  members  of  the  two  divisions  to  the 
solvent  action  of  white  spirit.  It  looks  promising  in  spite 
of  several  exceptions  due  to  the  variety  of  conditions  in 
the  production  of  these  complex  bodies.  Lawton  {loc.  cit.) 
gives  the  results  of  the  examination  of  a  number  of  pitches, 
especially  stearine  pitches. 

The  natural  asphaltums  or  bitumens  containing  sulphur 
and  nitrogen  can  be  separated  into  petrolenes,  soluble  in 
88°  C.  petrol,  and  asphaltenes.  Petrolenes  are  tough  viscid 
substances  possessing  cementing  properties,  whereas  the 
asphaltenes  are  dry  and  brittle,  but  soluble  in  carbon  disul- 
phide  and  in  hot  turpentine. 

Petrol  (88°  C.)  dissolves  coal-tar  or  wood-tar  pitches 
which  would  be  used  in  varnish  enamels,  but  the  petrolenes 
are  unsaponifiable  by  alkalies. 

The  discussion  of  the  origin  of  the  bitumen  deposits  is 
outside  the  scope  of  this  book.  As  hydrocarbons,  e.g.  bitu- 
men of  the  Dead  Sea  and  Trinidad  asphalt,  they  contain 


RESINS  AND  PITCHES 


107 


carbon,  hydrogen,  oxygen,  nitrogen,  and  sulphur,  and  yield 
on  distillation  gas,  coke,  burning  oils,  and  gas  oils,  so  that 
they  are  comparable  with  petroleum  pitches  produced 
artificially. 

Dead  Sea  asphalt.  Trinidad  asphalt. 


Carbon  . .  . .  77*8  78-8 

Hydrogen  . .  . .      8*93  9-3 

Nitrogen  . .  . .      —  1*4 

Sulphur  . .  . .      —  10 'o 

Oxygen  ..  ..  11-54  — 


100*54  99-5 

The  pitch  lakes  of  Trinidad  and  Bermudez  represent 
important  deposits  of  natural  bitumen,  but  their  origin, 
like  that  of  the  mineral  oils,  remains  undecided. 

Richardson  {J,  I nd.  Eng.  Chem.,  igi6,  ^,4)  suggests  that 
the  petroleum  pitches  originate  by  surface  action  between 
natural  gases  and  the  sands  with  which  they  come  in  con- 
tact, and  that  asphalts  are  formed  by  the  surface  action  of 
colloidal  clays  upon  heavy  petroleums.  Peckham  (/.  Frank, 
Inst.,  146,  (i)  45),  from  a  study  of  the  Californian  bitumens, 
considers  that  the  polymerization  of  petroleum  and  the 
conversion  into  asphalt  is  due  largely  to  the  presence  of 
nitrogen  and  sulphur. 

For  full  information  on  the  examination  of  pitches 
reference  may  be  made  to  the  following  works  : — 

Allen  :  "  Commercial  Organic  Analysis  :  "  Thorpe's  Dic- 
tionary of  Applied  Chemistry    (Pitches) .  Lunge  and  Keane  : 

Technical  Methods  of  Chemical  Analysis."  Ingle:  -'A 
Manual  of  Oils,  Resins,  and  Paints.''  Gardner  and 
Schseffer  :    The  Analysis  of  Paints."  Seeligman  and  Zieke  : 

Handbuch  der  I^ack  u.  Firnis  Industrie  "  (1914). 


Part  IV.— PIGMENTS,  PAINTS,  AND 
LINOLEUM 


Section  L— PIGMENTS  AND  PAINTS 

Pigments 

The  subject  of  pigments  introduces  a  primary  consideration 
of  their  uses  in  paints  and  enamels.  Their  employment 
as  a  means  of  obscuration  of  the  undercoat  for  decora- 
tive treatment  being  obvious,  the  question  of  choice  of 
paint  or  varnish  to  serve  as  a  protective  coat  leads  to  a 
consideration  of  the  general  influence  produced  by  a  pigment 
in  a  paint.  In  applying  a  drying  oil  medium  to  a  non- 
absorbent  surface,  the  thickness  of  film  applied  is  limited 
by  mainly  one  factor,  that  of  viscosity,  and  it  is  obvious 
that  such  is  limited  by  considerations  of  practicability. 
The  application  of  a  thick  film,  however,  can  be  realized 
by  applying  it  in  the  form  of  a  two-phase  system  of  solid/ 
liquid  wherein  the  rigidity  of  the  system  is  out  of  all  pro- 
portion greater  than  its  corresponding  viscosity  or  resistance 
to  shear.  The  net  result  of  such  is  to  furnish  a  means  of 
applying  a  heavier  or  thicker  coating  than  would  otherwise 
be  possible  in  the  absence  of  the  pigment.  There  are  other 
influences  at  work  in  a  paint  in  addition  to  these  which 
will  be  considered  under  the  heading  of  paint  and  enamels. 

The  several  properties  of  the  different  pigments  will  be 
considered  under  their  respective  headings,  but  a  review  of 
the  general  properties  of  pigments  used  in  paints  will  be 
necessary  at  this  juncture.  Pigments  may  have  one  or 
many  of  the  following  properties  : — 

io8 


PIGMENTS  AND  PAINTS  109 

(i)  Colour,  Opacity,  and  Tinctorial  power.  Although 
white  is  usually  not  considered  as  a  colour,  the  fact  that  its 
function  in  paints  is  often  to  overcome  or  kill another 
colour  makes  it  expedient  to  consider  it  as  such.  Tinctorial 
power  is  to  be  distinguished  from  the  other  property  of 
pigment  referred  to  as  colour  by  the  capacity  for  staining 
or  imparting  hue.  The  term  capacity  or  body  is 
more  strictly  accurate  in  referring  to  that  property  of  a 
pigment  which  is  distinguished  by  the  capacity  for  obscura- 
tion of  an  underground  or  undercoat.*  A  sharp  classifica- 
tion into  staining  and  obscurative  pigments  is  not  possible, 
as  certain  pigments  possess  both  properties,  and  the  property 
of  obscuration  or  opacity,  although  nearly  absent  in  certain 
pigments,  is  only  present  to  any  degree  in  others  under 
certain  conditions.  The  main  factor  in  obscuration  is  that 
of  producing  optical  discontinuity  in  the  film  under  con- 
sideration. This  clearly  resolves  itself  into  the  attainment 
of  a  layer  or  layers  through  which  the  light  passing  suffers 
different  degrees  of  refraction.  In  other  words,  high 
opacity  in  a  film  is  obtained  by  a  combination  of  pigment  and 
medium  of  refractive  indices  as  far  apart  as  possible.  The 
medium  may  be  oil,  water,  or  air,  so  that  it  may  happen 
that  two  pigments  may  have  nearly  the  same  opacity, 
examined  dry  or  in  water,  but  by  virtue  of  a  closer  ap- 
proximation of  refractive  index  of  linseed  oil  to  that  of  one 
of  the  pigments,  such  may  prove  more  or  less  transparent 
in  an  oil  medium.  The  higher  the  refractive  index  of  a 
pigment,  therefore,  the  greater  its  opacity,  since  no  medium 
exists  which  has  a  greater  index  of  refraction  than  that  of 
the  most  transparent  of  pigments.  The  ultimate  size  of  the 
particles  also  has  a  great  influence  on  the  opacity  of  a 
pigment,  the  smaller  the  size  of  the  pigment  the  higher  the 
opacity,  this  latter  property,  however,  diminishing  as  ap- 
proximation in  size  to  the  wave-length  of  light  is  approached. 

Tinctorial,  or  staining  power  is  the  relative  degree  to 
which  unit  weight  of  pigment  will  confer  colour  to  another 

*  For  a  description  of  an  ingenious  laboratory  apparatus  for  determining 
opacities  of  pigments,  see  Pfund;  /.  Franklin  Inst.,  1919,  188,  675-681. 


no    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


to  obtain  the  same  degree  of  tone  value.  Thus  two  samples 
of  Prussian  blue  will  be  compared  for  staining  power  by 
determining  the  relative  amounts  of  white  pigment  necessary 
to  obtain  pale  blues  of  similar  tone  or  light  reflecting 
value.  The  converse  of  this  property  of  staining  power, 
sometimes  referred  to  as  "  killing  power/'  represents  the 
result  of  a  similar  determination  on  a  white  pigment,  and 
the  procedure  is  self-evident.  The  two  properties  of  opacity 
and  staining  power  do  not  necessarily  bear  any  relationship 
towards  each  other,  though  usually  a  close  relationship 
sometimes  exists  between  opacity  and  killing  power  "  of 
white  pigments.  Thus  Prussian  blue  represents  a  pigment 
of  high  staining  power  and  very  low  opacity,  Indian  red 
possesses  both  great  staining  power  and  high  opacity, 
whilst  the  chromes  possess  moderate  staining  power  and 
high  opacity.  The  high  opacity  of  most  black  pigments 
is  probably  accounted  for  by  their  power  of  absorbing  light 
to  a  great  degree  as  well  as  by  their  high  state  of  subdivision. 

The  colour  of  pigments  often  varies  to  a  great  degree 
according  to  their  method  of  preparation.  Whilst  com- 
paratively small  variations  are  found  in  certain  classes  of 
pigments  such  as  Prussian  blue,  great  variations  in  hue 
are  to  be  found  among  the  earth  colours  and  chrome 
pigments. 

The  measurement  and  registration  of  colours  in  numerical 
terms  has  been  attempted  by  many  investigators.  One 
of  the  best-known  instruments  is  that  of  J.  W.  lyovibond, 
known  as  the  "  Tintometer.''  In  this  instrument  a  flattened 
heap  of  the  pigment  under  examination  is  viewed  simultane- 
ously with  a  similar  heap  of  calcium  sulphate  (taken  as  the 
standard  of  whiteness)  through  two  long  tubes  arranged 
side  by  side  as  binoculars,  glasses  of  standard  degrees  of 
depth  in  yellow,  blue,  and  red  being  interposed  in  the  tube 
through  which  the  calcium  sulphate  is  viewed  until  a  colour 
match  is  obtained.  It  is  thus  obvious,  from  a  consideration 
of  the  theory  of  colours,  that  theoretically  not  only  would 
it  be  possible  to  match  the  hue  of  a  given  colour,  but  in 
virtue  of  subtraciive  effect  obtained  by  passing  white  light 


PIGMENTS  AND  PAINTS  iii 

through  complete  and  equal  units  of  primary  colours  a 
certain  degree  of  obscuration  or  units  of  black  may  be 
obtained.  Thus,  if  a  given  colour  were  matched  by  inter- 
posing in  the  calcium  sulphate  tube  units  represented  by 
red  I '3,  yellow  07,  and  blue  3*0,  the  effect  produced  will 
be  equivalent  to  07  units  of  black,  o*6  units  of  red,  and 
2*3  units  of  blue.  Unfortunately,  however,  the  instrument 
has  not  found  much  favour  among  workers  in  pigments,  as 
the  difficulty  of  obtaining  colour  matches  of  richly  coloured 
pigments  is  exceedingly  difficult  owing  probably  to  fatigue 
of  the  eyes.  It  is  understood,  however,  that  for  tintings  of 
white,  such  as  would  occur  in  white  pigments,  flours,  pale 
coloured  aqueous  extracts,  etc.,  the  instrument  has  been 
found  very  valuable.  Other  methods  of  colour  measure- 
ment *  have  been  suggested,  but  being  of  comparatively 
recent  introduction,  nothing  yet  is  known  as  to  their  value 
in  practice. 

An  important  point  in  connection  with  the  valuation 
of  pigments  in  regard  to  their  colour  arises  in  their  so-called 
undertone  "  obtained  on  reduction.  This  undertone  often 
appears  as  a  colour  complementary  to  that  which  dominates 
in  the  pure  pigment.  Thus,  although  chrome  yellows,  yellow 
ochres,  red  oxides,  and  many  colours  on  dilution  with  white 
pigments  yield  colours  of  lesser  purity  but  of  similar  hue, 
others,  such  as  many  lake  reds,  browns,  etc.,  yield  under 
similar  conditions  colours  of  lesser  purity  dominating  in 
hue.  The  apparently  dead  black  pigment,  vegetable  black, 
yields  on  reduction  greys  with  a  strong  blue  cast,  whilst 
carbon  blacks  yield  brown-greys. 

(ii)  Chemical  Effect  upon  the  Medium,  In  spite  of  its 
many  disadvantages,  the  great  popularity  of  white  lead,  or 
basic  carbonate  of  lead,  as  a  pigment  is  undoubtedly  due 
to  its  effect  upon  the  medium  (linseed  oil)  in  which  it  is 
used.  It  stands  by  no  means  at  the  head  of  the  list  of  white 
pigments  for  either  colour  (purity  of  its  white),  opacity, 
or  cost,  yet  it  is  safe  to  assume  that  it  finds  its  way  into 

*  Lawrance,  O.  and  Col.  Chem.  Assocn.,  1919,2,  No.  6;  Bawtree,  0,  and 
Col.  Chem,  Assocn.,  1919,  2,  61. 


112    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


more  paints  than  any  other  pigment.  It  being  a  recognized 
fact  that  its  basicity  is  a  cause  of  a  slight  degree  of  saponifi- 
cation of  the  oil  medium,  it  is  probable  that  the  small 
amount  of  lead  soap  formed  is  responsible  for  the  enhanced 
physical  properties  of  paints  of  which  it  is  a  component. 
The  comparative  impermeability  of  this  lead  soap  is  also  a 
probable  factor  in  its  extended  use. 

Among  other  pigments  having  decided  chemical  effect 
on  the  medium  may  be  cited  those  which  affect  it  by  reason 
of  their  basicity  or  power  to  neutralize  nascent  lower  fatty 
acids  (such  as  formic  and  acetic  acids,  etc.,  from  boiled  oil 
during  oxidation),  and  those  influencing  the  oxidation 
either  favourably  by  virtue  of  their  containing  a  compound 
of  a  metal  which  acts  catalytically  as  a  drier,''  e.g.  certain 
ochres,  umbers,  etc.,  or  unfavourably  by  reason  of  deleterious 
impurities.  These  will  be  considered  under  the  separate 
headings  of  the  pigments,  although,  in  many  cases,  the 
influence  of  retardation  of  oxidation  is  not  well  understood. 

(iii)  Physical  Effect  upon  the  Paint.  An  important  point 
leading  eventually  to  marked  influences  on  the  behaviour 
of  the  paint  film  on  exposure  is  that  of  the  relative  oil 
absorption  of  pigments,  and  will  be  considered  more  fully 
when  dealing  with  the  subject  of  the  preparation  of  paints 
and  enamels.  The  oil  absorption  of  pigments  may  be 
defined  as  that  minimum  quantity  of  oil  necessary  to  convert 
unit  weight  of  dry  pigment  from  a  powder  to  a  definite 
paste.  This  constant  is  conveniently  determined  in  the 
laboratory  by  adding  raw  linseed  oil  drop  by  drop  from  a 
burette  to  (say)  lo  grams  of  pigment  in  a  mortar  and 
working  in  the  oil  with  a  powerful  grinding  action  until  a 
completely  coherent  stiff  paste  is  obtained.  The  oil  absorp- 
tion of  pigments  varies  from  some  6  per  cent,  in  the  case 
of  white  lead  to  nearly  200  per  cent,  in  the  case  of  certain 
smoke-black  pigments. 

The  ultimate  physical  condition  of  a  pigment,  i.e.  the 
question  of  its  fineness  of  division,  specific  gravity,  and 
electrical  charge,  which  it  carries  when  suspended  in  a 
particular  medium,  or  in  other  words,  its  approach  to  01 


PIGMENTS  AND  PAINTS 


113 


departure  from  the  colloidal  state,  has  a  very  great  bearing 
on  its  employment  in  paints,  and  even  more  so  in  enamels. 
To  cite  the  case  of  pigments,  which  exist  in  media  to 
a  great  extent  in  the  colloidal  state,  there  is  the  case  of 
zinc  oxide  in  enamel  media.  These  latter  usually  consist 
of  thickened  oils  or  varnishes  very  rich  in  thickened  oils. 
The  particular  combination  of  actively-basic  zinc  oxide  in 
a  finely  divided  state,  together  with  oils  of  moderate  acidity 
and  high  viscosity,  results  in  a  colloid  system  of  very 
great  stability.  Thus,  separation  of  pigment  from  medium, 
even  when  diluted  with  ether,  cannot  be  accomplished 
by  ordinary  filtration,  and  in  an  experiment  by  one 
of  the  writers,  complete  sedimentation  did  not  take  place 
in  an  ethereal  dilution  in  eighteen  months,  the  high-speed 
centrifuge  alone  effecting  complete  deposition  with  some 
difficulty.  The  suspension  showed  all  the  attributes  of  a 
suspensoid,  the  Tyndall  cone  effect  being  quite  marked,  and 
rapid  Brownian  movement  being  visible  in  the  ultramicro- 
scope.  The  suspensoid,  moreover,  was  bluish  by  reflected, 
and  orange  by  transmitted  light.  A  solution  of  an 
intimately  ground  mixture  of  linoxyn  and  Peruvian  ochre  in 
amyl  alcohol  showed  the  pigment  to  be  in  a  similar  condition. 

The  colloidal  condition  of  smoke-black  in  the  preparation 
known  as  Indian  ink is  well  known,  the  aqueous  solution 
absolutely  defying  separation  of  the  pigment  by  the 
ordinary  means  of  filtration.  Perhaps  the  most  perfect  case 
of  colloidal  suspension  is  that  of  Prussian  blue  in  oil.  In 
this  case  the  high-speed  centrifuge  is  quite  unable  to  remove 
the  last  traces  of  suspended  pigment. 

A  property  often  utilized  in  pigments  and  probably 
intimately  linked  up  with  the  foregoing,  may,  for  want  of  a 
better  name,  be  termed  suspending  power.''  This  is  well 
exemplified  in  the  case  of  China  clay,  which,  although  not 
showing  the  usual  attributes  of  a  colloidally-suspended  pig- 
ment, is  nevertheless  capable  of  preventing  deposition  to  a 
hard  cake  in  the  bottom  of  the  container  in  the  case  of  paints 
containing  pigments  of  known  high  specific  gravity  and  low 
oil  absorption.  Common  whiting  similarly  shows  this 
s,  8 


114    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


property,  whilst  precipitated  chalk,  which  for  all  practical 
purposes  is  of  similar  composition,  does  not  act  thus. 

A  substance  possessing  particular  form  of  its  particles 
in  the  finely  divided  condition  is  often  useful  to  secure 
certain  physical  characteristics  in  the  dry  coating.  White 
lead  examined  microscopically  is  to  a  certain  degree  crystal- 
line, and  this  property  imparts  bite  or  tooth  to  white 
lead  paints,  both  on  application  and  for  the  subsequent  coat. 

The  White  Pigments 

I.  White  Lead  or  Basic  Lead  Carbonate. — White 
lead,  often  incorrectly  termed  carbonate  of  lead,  is  perhaps 
the  best  known  and  most  important  of  the  white  pigments. 
The  earliest  process  of  manufacture,  that  known  as  the 
stack  "  or  Dutch  process,  is  the  one  which,  at  all  events, 
until  quite  recently,  enjoyed  the  greatest  popularity,  although 
the  time  taken  and  difficulties  of  manipulation  have  proved 
a  great  incentive  to  attempts  to  introduce  more  rapid  and 
certain  methods  of  manufacture,  as  will  be  judged  by  the 
extensive  patent  literature  dealing  with  the  subject.  In 
brief,  the  stack  method  of  corrosion  consists  in  forming  a 
stack  of  earthenware  vessels,  each  having  a  false  bottom 
acting  as  a  container  for  dilute  acetic  acid  or  vinegar,  and 
supporting  on  the  ledge  formed  by  the  false  bottom  a  roll  of 
thin  sheet  lead.  The  stack  or  battery  of  pots  is  then  enclosed 
in  a  chamber  over  spent  tan  or  dung  for  a  period  of  about  two 
months,  during  which  time  corrosion  of  the  lead  to  basic 
acetate  takes  place  by  means  of  the  acetic  acid  volatilized 
by  the  heat  of  the  fermenting  organic  matter,  followed 
by  its  conversion  into  basic  lead  carbonate  by  the  carbonic 
acid  evolved.  The  stack  is  then  opened,  the  corroded  grids 
removed,  and  the  unattacked  blue lead  separated  by 
crushing,  washing,  etc.,  the  metal-free  pigment  being  dried 
in  the  usual  way.  The  main  disadvantages  of  the  process 
are  the  length  of  time  occupied  for  corrosion,  the  danger  to 
the  operatives  opening  the  stack,  and  the  liability  to  con- 
tamination by  lead  sulphide  from  the  sulphuretted  hydrogen 


PIGMENTS  AND  PAINTS  115 


evolved  from  the  organic  matter.  On  the  other  hand,  the 
uniformity  of  the  product  in  so  far  as  its  comparatively 
amorphous  condition,  opacity,  etc.,  is  concerned,  has  for 
long  placed  the  product  made  by  this  method  in  the  front 
rank  of  excellence. 

Many  other  processes  have  been  devised  to  shorten 
the  period  of  corrosion,  whilst  attempting  to  retain  the 
fineness  of  division  of  the  stack-made  product.*  Although 
the  plant  used  differs  in  the  individual  processes,  the  opera- 
tions involved  are  nearly  all  alike,  depending  as  they  do 
on  solution  of  either  "blue''  lead  or  litharge  in  nitric,  or, 
more  usually,  acetic  acid,  and  subsequent  precipitation  with 
carbonic  acid  gas.  An  interesting  instance  of  a  precipita- 
tion "  process  is  that  of  Bischof,  as  now  carried  out  by  the 
Brimsdown  lycad  Co.  Ltd.,  at  Bfimsdown,  near  London. 
In  this  process  litharge  is  first  reduced  to  the  state  of 
the  black  sub-oxide  Pb20  by  means  of  water-gas  obtained 
by  blowing  steam  through  charcoal.  This  is  in  order  to 
ensure  reduction  of  any  red  lead  which  might  exist  in  the 
litharge,  and  which  would  eventually  pass  through  the 
processes  to  the  white  lead,  giving  it  a  pink  cast.  The  sub- 
oxide is  then  pugged  "  or  masticated  with  water  to  convert 
it  into  the  white  hydrate,  after  which  it  is  treated  in  a  large 
closed  wooden  vat  with  a  solution  of  lead  acetate  and  a 
little  acetic  acid,  carbonic  acid  meanwhile  being  pumped 
in  under  pressure.  The  amount  of  gas  introduced  is 
measured  and  serves  to  produce  a  basic  lead  carbonate 
of  any  degree  of  basicity.  The  acetic  acid,  serving  only  as 
a  catalyst,  can  be  recovered,  and  is  used  over  and  over 
again  for  succeeding  operations. 

Of  late  years  perfection  of  control  has  reached  such  a 
stage  that  white  lead  products  made  by  quick  processes 
compare  very  well  indeed  with  the  stack-made  article,  and 
which  are  far  superior  from  the  point  of  purity  of  colour. 

White  lead,  although  varying  somewhat  in  composi- 
tion in  different  samples,  approximates  to  the  formula 
Pb(OH)22PbC08.    This  formula  would  be  represented  by 

*  Hurst,  "  Painters'  Colours,  Oils,  and  Varnishes/'  5th  edition 


ii6   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


a  compound  containing  about  70  per  cent,  of  lead  carbonate 
and  30  per  cent,  of  hydrate,  whilst  the  average  of  a  number 
of  samples  approximates  to  72  per  cent,  of  the  former  to 
28  per  cent,  of  the  latter.  Dry  white  lead  comes  on  the 
market  in  the  form  of  a  heavy  white  powder  or  soft  lumps, 
possessing,  according  to  many  authorities,  a  faint  but 
characteristic  odour.  It  has  a  specific  gravity  of  6*6,  and 
an  oil  absorption  of  as  low  as  6  per  cent,  in  some  samples, 
i,e.  a  definite  paste  can  be  formed  by  combination  of  94  parts 
by  weight  of  dry  white  lead  with  6  parts  by  weight  of 
raw  linseed  oil.  The  production  of  the  stiff  product,  com- 
monly known  as  white  lead  paste  or  white  lead  in  oil,'' 
can  either  be  made  by  directly  grinding  the  dry  pigment  in 
oil  or  by  pugging  "  or  milling  the  wet  pulp  as  it  comes 
from  the  filter  press  with  linseed  oil,  the  superior  afiinity  of 
white  lead  for  oil  driving  out  the  water.* 

White  lead  is  a  pigment  of  high  opacity  and  fairly  good 
stability.  Its  great  popularity  in  paints  is  principally  due, 
however,  to  its  power  of  combining  to  a  certain  extent  with 
the  oil  medium  used,  the  lead  linoleate  formed  conferring 
certain  physical  properties  of  flow  under  the  brush  which 
are  highly  esteemed  in  practice,  and  a  good  degree  of 
impermeability  to  moisture.  White  lead  enters  into  the 
composition  of  many  paints  as  a  base  or  starting  point 
on  account  of  its  high  opacity  and  comparative  stability, 
many  pale-coloured  tinted  paints  being  obtained  therefrom 
by  addition  of  small  proportions  of  stainers. 

The  stability  of  white  lead  in  paints  has  formed  the 
subject  of  much  controversy  of  recent  years,  no  very  satis- 
factory agreement  or  conclusions  having  been  arrived  at. 
In  the  writers'  opinion,  however,  this  would  seem  to  be 
accounted  for  to  a  very  great  extent  by  the  fact  that  too 
little  attention  seems  to  have  been  paid  to  the  other  factors 
obtaining  in  the  paint  under  trial,  i.e.  the  medium  used  and 
its  relative  amount,  as  it  is  obvious  that  pigmentation  or 

*  The  cause  of  this  phenomenon  is  to  be  attributed  to  the  lower  inter- 
facial  tension  existing  in  white  lead/oil  over  that  of  white  lead/water 
resulting  in  a  diminution  of  surface  energy. 


PIGMENTS  AND  PAINTS 


117 


relative  proportion  of  pigment  to  oil  in  a  given  medium 
beyond  a  limit,  is  likely  to  lead  to  actual  exposure  of  the 
individual  pigment  particles  to  a  certain  extent.  From 
the  mass  of  controversial  evidence,  however,  a  few  definite 
facts  stand  out.  Of  all  the  white  pigments  white  lead  is 
that  which  has  the  greatest  tendency  to  go  yellow  in  the 
dark.  This  is  not  a  very  serious  disadvantage,  as  the  3^ellow- 
ness  bleaches  out  once  more  when  the  paint  is  exposed  to 
a  good  light.  White  lead  paint  is  also  very  susceptible  to 
blackening  in  an  atmosphere  containing  sulphuretted  hydro- 
gen. The  majority  of  observers  also  seem  to  agree  that  on 
exposure  to  much  sunlight  and  weather  white  lead  paint 
films  soon  lose  their  gloss  and  chalk,"  i.e,  the  oil  medium 
seems  to  disappear,  leaving  the  pigment  in  an  unbound 
chalky  condition.  This  latter  is  probably  due  to  the 
catalytic  action  of  the  lead  as  a  drier,  drying,  as  already 
stated,  being  a  reaction  by  no  means  ended  when  solidity 
of  the  paint  film  has  once  been  attained. 

Of  the  numerous  other  white  lead  pigments  which  have 
engaged  transitory  attention  from  time  to  time  may  be  cited 
the  basic  chloride,  hydrate,  sulphite,  etc.,  the  only  one  which 
has  found  anything  like  a  moderate  degree  of  popularity 
being  the  basic  sulphate. 

2.  Basic  Lead  Sulphate  or  Sublimed  White  Lead. — 
This  product,  as  found  on  the  market,  varies  greatly  in  basicity, 
some  samples  having  as  low  a  PbO  content  as  i  or  2  per  cent,, 
whilst  others  contain  up  to  35  per  cent.  The  usual  method  of 
manufacture  is  by  sublimation  of  galena,  PbS,  in  a  current 
of  air.  It  is  also  formed  as  a  by-product  in  the  manufacture 
of  litharge  by  cupellation,  a  certain  proportion  of  lead  com- 
pound being  volatilized  by  the  action  of  the  heating  gases 
containing  sulphur  dioxide.  A  method  of  only  academic 
interest  consists  in  the  precipitation  of  the  normal  sulphate 
from  a  soluble  lead  salt  and  subsequent  grinding  of  the 
precipitate  in  a  solution  of  a  caustic  alkali,  when  a  definite 
reaction  with  increase  of  temperature  accompanies  the  forma- 
tion of  the  basic  sulphate.  No  specific  claim  for  distinction 
from  white  lead  in  properties  seems  to  have  been  ^iade» 


ii8    RUBBER.  RESINS,  PAINTS  AND  VARNISHES 


barring,  perhaps,  the  altogether  erroneous  one  of  its  absence 
of  toxicity.  Generally  speaking,  it  may  be  said  that  the  con- 
sumption of  basic  lead  sulphate  as  a  pigment  is  very  limited. 

3.  Zinc  Oxide  or  Zinc  White.— This  pigment  is  made 
by  two  different  methods,  known  as  the  direct  and  the 
indirect  processes.  The  former,  also  known  as  the  French 
process,  consists  in  burning  metallic  zinc  in  a  current  of 
air,  the  zinc  oxide  or  fume  being  condensed  in  chambers 
containing  bags.  Indirect  process  oxide  is  obtained  by  burn- 
ing the  zinc  ore,  the  product  differing  from  that  from  the 
other  process  by  its  lesser  purity  of  composition  and  colour. 

Zinc  oxide  is  the  whitest  of  all  commercial  pigments, 
ranking  slightly  behind  precipitated  calcium  sulphate  in 
its  purity  of  colour.  Zinc  oxide  is  also  the  white  pigment 
possessing  the  highest  degree  of  subdivision,  this  property 
making  it  the  most  suitable  pigment  to  use  in  white  enamels. 
Owing  to  the  actively  basic  nature  of  zinc  oxide,  much  care 
has  to  be  exercised  in  the  selection  of  the  medium  used  in 
its  grinding,  a  badly-refined  oil  of  high  acid  value  being 
unsuitable  by  reason  of  the  stiffening  or  livering  "  which 
occurs  progressively  after  grinding.  The  opacity  of  zinc 
oxide  is  equal  to  or  possibly  greater  than  that  of  white  lead, 
but  by  reason  of  its  greater  oil  absorption  (15  per  cent,  as 
against  6  per  cent.)  the  opacity  of  a  zinc  white  paint  appears 
less  than  that  of  white  lead  paint  on  account  of  the  lower 
pigmentation  obtaining  in  the  former  (H.  Pfund,  /.  Franklin 
Inst.,  1919,  iSS,  675-681).  On  the  other  hand,  the  spreading 
power  or  relative  area  covered  by  unit  weight  of  paint  in 
the  case  of  zinc  white  is  much  greater  than  with  white  lead. 
The  stability  of  zinc  white  in  paint  films  is  good,  its  higher 
cost  usually  being  considered  as  one  of  the  factors  mitigating 
against  its  displacing  white  lead.  In  its  favour  may  be 
stated  its  absolute  non-toxicity  and  its  power  to  remain 
well  suspended  in  paints. 

4.  Orr's  White  or  Lithopone. — This  pigment  was  first 
manufactured  and  patented  in  1874  by  Mr.  J.  B.  Orr,  It 
is  one  of  the  most  interesting  of  the  white  pigments,  and 
now  holds  an  important  position  in  the  paint  factory. 


PIGMENTS  AND  PAINTS 


119 


where  it  has  practically  supplanted  every  other  pigment 
for  use  in  indoor  paints  and  enamels.  lyithopone,  also 
known  under  a  variety  of  other  names,  e.g.  ponolith,  litho- 
phone,  enamel  white,  etc.,  consists  of  approximately  mole- 
cular proportions  of  zinc  sulphide  and  barium  sulphate,  but 
in  so  far  that  the  product  is  subjected  to  calcination  during 
the  course  of  its  manufacture,  small  proportions,  1-5  per  cent, 
of  zinc  oxide  are  usually  present.  Lithopone,  therefore,  has 
the  average  composition:  ZnS  30*0  per  cent.,  ZnO  I'c  per 
cent.,  BaS04  69*o  percent.  It  is  prepared  by  double  decom- 
position of  solutions  of  zinc  sulphate  and  barium  sulphide, 
the  latter  being  obtained  by  calcination  of  barytes  with 
coal  or  other  carbonaceous  matter.  The  precipitate  is 
filtered,  washed,  and  dried,  but  to  increase  its  density  and 
opacity  and  to  reduce  its  oil  absorption,  it  is  subsequently 
calcined  to  a  white  heat  in  crucibles  in  the  presence  of 
air-excluding  agents,  e.g,  sulphur  or  ammonium  chloride, 
and  finally  chilled  in  cold  water.  The  pigment  is  then 
filter-pressed,  washed,  and  dried. 

Since  more  than  two-thirds  of  the  pigment  consists  of 
barium  sulphate,  a  body  of  low  opacity  and  low  oil  absorp- 
tion, it  might  at  first  sight  be  regarded  as  a  diluent  or  adul- 
terant, diluting  the  valuable  properties  of  the  zinc  sulphide. 
But  inasmuch  as  the  zinc  sulphide  is  precipitated  along 
with  the  barium  sulphate,  quite  a  different  product  is  obtained 
from  that  prepared  by  mixing  the  two  constituents  together, 
and  indeed  lithopone  actually  possesses  a  higher  opacity 
than  any  known  white  pigment.  It  is  generally  considered 
that  the  particles  of  barium  sulphate  are  adsorbed  on  the 
individual  zinc  sulphide  particles,  thus  giving  it  enhanced 
properties  over  that  of  its  constituents. 

lyithopone  has  a  specific  gravity  about  4*25,  and  an  oil 
absorption  of  about  8  per  cent.  As  already  stated,  it  is  the 
most  opaque  of  all  white  pigments,  whilst  its  staining  or 
"  killing  "  power  is  considerably  greater  than  that  of  white 
lead.  It  is  quite  non-poisonous,  a  property  which  befits 
it  particularly  for  use  in  paints  for  toys.  In  addition,  it 
possesses  little  or  no  basicity,  and  it  can  in  consequence  be 


120   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


used  without  fear  of  feeding with  the  most  acid  media ;  in 
fact,  it  is  quite  stable  when  used  with  a  medium  consisting 
mainly  of  rosin.  The  fineness  or  subdivision  of  lithopone 
is  about  equal  to  that  of  white  lead. 

The  weather  resistance  of  lithopone  in  paints,  however, 
leaves  much  to  be  desired,  and  present-day  practice  has 
practically  unanimously  condemned  its  use  in  paints  for 
outdoor  work,  failure  of  the  paint  film  occurring  very  early. 
Opinion  is  divided  as  to  whether  this  failure  is  due  to  vulcani- 
zation of  the  oil  in  the  medium  by  the  agency  of  the  zinc 
sulphide  with  consequent  progressive  loss  of  elasticity,  or 
oxidation  of  the  zinc  sulphide  to  soluble  sulphite  or  sulphate 
with  resultant  disintegration  of  the  film.  An  important  draw- 
back to  the  use  of  lithopone,  especially  where  the  article 
coated  is  exposed  to  much  direct  sunlight,  is  the  tendency 
of  many  samples  to  darken  under  these  conditions.  This 
darkening  is  not  permanent,  but  disappears  when  the  source 
of  light  is  removed  for  some  little  time,  only  to  return  on 
repeated  exposure.  No  satisfactory  explanation  of  this 
phenomenon  has  been  advanced,  but  it  is  recognized  that 
the  change,  whatever  it  is,  is  produced  in  the  zinc  sulphide 
part  of  the  product.  Moisture  is  essential  for  its  appearance, 
carefully  dried  samples  sealed  in  glass  tubes  being  quite 
stable.  Many  patented  processes  claim  to  produce  litho- 
pone having  no  tendency  to  alter  in  sunlight,  and  it  is  only 
fair  to  state  that  many  manufacturers  are  now  producing 
this  pigment  without  its  usual  attendant  disadvantage, 
lyithopone  finds  its  principal  use  in  the  paint  industry  in 
interior  paints  and  enamels  and  in  water-paints  (distemper) . 
By  far  the  largest  outlet,  however,  is  in  the  rubber  and 
linoleum  industries,  its  stability  at  the  temperature  of 
vulcanization  particularly  fitting  it  for  use  in  the  former, 
whilst  its  non-reactivity  to  acid  oxidized  oil  and  rosin  render 
it  useful  in  mixing  with  linoleum  cement  for    inlaid  work. 

The  F11.1.ERS  AND  Extenders 
These  are  substances  used  either  to  adulterate  paints 
for  the  purposes  of  cheapness  or  to  confer  some  special 


PIGMENTS  AND  PAINTS 


properties  other  than  that  of  tinting  or  opacity.  What 
might  at  first  sight  be  regarded  as  coming  under  the  head  of 
adulteration  is  often  necessary,  a  specific  example  of  which 
will  make  the  case  clearer.  Whilst  in  fine  painting  work, 
such  as  coach  painting,  the  body  or  thickness  of  colour 
applied  is  not  considered  when  applying  that  particular 
coat  in  which  coloration,  as  distinct  from  filling  or  bodying 
is  concerned,  the  house-decorator  needs  not  only  to  produce 
obscuration  of  his  under-ground  but  to  obtain  thickness  by 
applying  the  minimum  number  of  coats  of  paint.  Taking 
the  case  of  such  a  colour  as  lamp-black,  chosen  in  particular 
on  account  both  of  its  high  tinctorial  power,  high  opacity, 
and  high  oil  absorption,  the  use  of  a  pure  or  genuine 
paint  would  hardly  be  practicable  where  the  above  case  is 
considered,  as  either  the  paint  would  have  such  a  consistency 
as  to  be  almost  a  jelly,  or,  in  the  case  of  a  suitable  consistency 
being  obtained  by  low  pigmentation,  the  paint  film  would 
be  too  attenuated.  Thus  an  inert  filler,  such  as  common 
whiting  or  barytes,  is  used  in  order  that  the  physical  pro- 
perties of  the  paint  may  approach  those  of  the  more  normal 
pigments,  the  quantity  of  black  pigment  employed  being 
merely  sufficient  to  secure  adequate  pigmentation  and  opacity. 

Another  case  of  employment  of  inert  pigments  or 
extenders  is  that  of  the  so-called  lake  pigments.  In  lake 
pigments  the  inert  filler  takes  precisely  the  same  place  as 
the  fabric  in  dyeing,  i.e.  it  serves  as  a  base  or  recipient 
for  the  lake,  without  which  the  lake  would  be  useless. 

Barytes  and  Blanc  Fixe. — These  two  fillers,  although 
as  similar  in  composition  as  a  natural  ore  and  an  artificial 
product  may  be,  differ  greatly  in  their  physical  properties. 
Common  barytes  consists  of  crushed  heavy  spar  (BaS04), 
which  has  been  levigated  or  water-floated  to  free  it  from 
coarser  particles.  In  cases  where  much  impurity  in  the 
form  of  iron  oxide  is  present  giving  the  barytes  a  yellow 
or  reddish  tint,  it  is  bleached  by  treatment  with  hot  dilute 
sulphuric  or  hydrochloric  acid,  the  purified  product  being 
subsequently  washed  and  dried.  The  colour  of  barytes 
should  be  carefully  attended  to  when  buying,  since  the 


122    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


pigment  finds  one  of  its  principal  applications  in  the  dilution 
or  reduction  of  white  lead.  The  writers  have  found  that 
many  samples  of  apparently  pure  colour  indicating  freedom 
from  iron  develop  yellowness  after  rubbing  with  linseed  oil 
and  allowing  to  stand  an  hour  or  two,  and  this  test  is 
recommended  when  a  determination  of  iron  is  not  considered 
expedient. 

The  specific  gravity  of  barytes  varies  from  4*2  to  4*5, 
and  the  oil  absorption  from  6  per  cent,  in  the  coarser  samples 
to  9  per  cent,  in  the  finest.  This  pigment  is  particularly 
inert,  both  chemically  and  physically,  there  being  practically 
no  tendency  to  pass  to  any  degree  into  colloidal  solution  in 
the  medium.  Hence  barytes  should  not  be  used  alone  with 
pigments  of  similar  tendency,  as  settlement  to  a  hard  cake 
in  the  bottom  of  the  container  is  likely  to  occur. 

The  inactivity  of  barytes  and  its  low  oil  absorption  places 
it  in  the  front  rank  of  importance  as  a  diluent  of  paints, 
since  the  majority  of  these  are  sold  by  weight.  It  is  the 
recognized  standard  for  reducing  or  diluting  white  lead 
in  the  trade,  the  various  reductions  being  known  as  No.  i 
White  Lead,  No.  2  White  lycad,  etc.,  according  to  the 
relative  proportions  of  genuine  white  lead  and  white  barytes 
used.  Although  commonly  regarded  as  nothing  more  nor 
less  than  an  adulterant,  a  moderate  proportion  of  barytes  is 
regarded  by  many  authorities  as  an  improvement  to  many 
paints,  the  crystalline  nature  of  the  particles  offering  a  good 
surface  for  repainting,  whilst  films  of  paint  containing 
barytes  suffer  less  contraction  as  they  age  than  is  the  case 
when  pure  white  lead  has  been  employed  as  the  pigment. 

Barytes  possesses  little  or  no  opacity,  and  is  practically 
deficient  in  "killing''  power,  hence  it  is  possible  to 
reduce  such  pigments  as  greens  with  barytes  with  little 
loss  of  purity  of  colour. 

Blanc  fixe,  or  precipitated  barium  sulphate,  differs  rather 
markedly  in  its  physical  properties  from  natural  barytes. 
It  is  prepared  by  precipitation  of  barium  chloride  by  sul- 
phuric acid.  Certain  samples  are  liable  to  be  imperfectly 
washed,  when  the  traces  of  free  sulphuric  acid  remaining 


PIGMENTS  AND  PAINTS 


123 


may  have  a  very  adverse  influence  on  the  paint  into  the 
composition  of  which  it  enters,  for  obvious  reasons.  The 
characteristic  differences  between  blanc  fixe  and  barytes  are 
the  amorphous  condition  and  finer  state  of  subdivision  of 
the  former.  These  manifest  themselves  in  greater  oil 
absorption  (15  per  cent.)  and  a  higher  degree  of  opacity. 
The  opacity,  however,  is  so  low  that  the  pigment  cannot  be 
employed  in  any  capacity  than  that  of  an  auxiliary  one, 
and  it  finds  its  principal  application  in  serving  as  a  base  for 
the  striking  of  dyes  to  lakes.  It  has  in  addition  a  useful 
field  of  application  in  reduction  of  pure  strong  colours 
such  as  ultramarine  and  Prussian  blues,  when  the  light- 
reflecting  power  or  visibility  of  the  hue  becomes  apparent 
without  manifestation  of  undertone  or  production  of 
muddiness."  This  property  is  most  probably  comparable 
to  reduction  of  pigmentation  in  an  oil/pigment  system 
without  simultaneous  reduction  in  solid  matter  content, 
i.e,  viscosity. 

Whiting. — Whiting  is  a  naturally-occurring  form  of 
calcium  carbonate  found  as  chalk  deposits  in  the  south- 
eastern counties  of  England.  Examined  microscopically,  it 
is  found  to  consist  of  the  skeleton  remains  of  various  species 
of  Fomminifera,  a  marine  organism.  Commercial  whiting 
consists  of  chalk  which  has  been  ground  and  levigated  in 
water.  It  has  a  specific  gravity  of  2*65.  Whiting  finds  little 
application  in  oil  paints  as  a  pigment  on  account  of  its  low 
opacity,  whilst  its  high  oil  absorption  (about  15  per  cent.) 
does  not  render  its  use  economical  as  a  filler.  It  finds  its 
principal  application,  however,  when  ground  to  a  stiff  paste 
in  linseed  oil  as  putty.  It  also  forms  a  valuable  pigment 
in  water-paints,  where  it  appears  to  function  as  a  pigment 
of  high  opacity  on  account  of  its  difference  of  refractive 
index  from  that  of  the  medium  present  in  the  dried  film. 

A  form  of  artificial  whiting  known  as  precipitated^chalk, 
or  under  its  trade  term  of  barytes  substitute,''  found 
considerable  application  during  the  war  when  barytes  was 
scarce,  and  was  produced  as  a  by-product  from  the  process 
of  water  softening.    This  precipitated  chalk  was  distinctly 


124    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


crystalline  in  structure,  of  better  colour  than  the  natural 
article,  and  of  rather  lower  oil  absorption. 

Alumina  or  Aluminium  Hydrate. — ^Alumina  is  pre- 
pared by  the  precipitation  of  ammonia  alum  or  aluminium 
sulphate  with  ammonium  hydrate,  the  gelatinous  precipitate 
being  filter-pressed,  washed,  and  dried.  It  forms  a  volumi- 
nous, white  powder,  which  is  exceedingly  transparent  when 
mixed  with  oil.  On  this  account  it  finds  little  or  no  applica- 
tion in  paint  pigments,  its  use  being  entirely  restricted  to 
pigments  for  use  in  printing  inks,  where  great  transparency 
is  desired.  It  is  there  used  as  a  base  for  precipitating  coal- 
tar  colours  to  form  lakes. 

China  Clay  or  Kaolin. — China  clay  is  a  hydrated  alumi- 
nium silicate  corresponding  approximately  to  the  formula 
Al2032Si022H20.  It  occurs  as  a  disintegration  product  of  the 
granite  rocks  of  Cornwall  in  this  country.  China  clay,  on 
account  of  its  high  oil  absorption  and  transparency,  is  of  little 
value  as  a  pigment  in  oil  paints,  nevertheless  on  account  of 
some  obscure  property  of  appearing  as  a  water-protected 
suspensoid  when  mixed  with  oil,  it  not  only  remains  in  good 
suspension,  but  inhibits  settlement  of  other  pigments,  e.g. 
barytes,  red  lead,  etc.  It  is  principally  used,  however,  as 
the  main  pigment  in  water-paints  or  distempers,  its  function 
as  an  electro-negative  colloid  in  aqueous  media,  the  stability 
of  suspension  of  which  is  increased  in  presence  of  alkalies 
(OH*  anions)  befitting  it  especially  as  an  ideal  pigment. 

China  clay  also  finds  application  as  a  base  for  the  pre- 
cipitation thereon  of  coal-tar  colours  intended  for  use  in 
transparent  oil  pigments  for  printing  inks  or  water-paints. 

Silica. — ^Although  this  substance  can  hardly  be  classed 
as  a  pigment,  it  enters  very  largely  into  the  composition 
of  many  types  of  paint  where  its  distinctly  crystalline 
structure  renders  it  especially  valuable  in  conferring 
tooth or  bite to  an  otherwise  smooth  or  soapy  pig- 
ment. The  most  valuable  varieties  of  silica  for  use  in  the 
paint  trade  are  obtained  by  crushing  quartz  and  fractionating 
into  different  degrees  of  fineness  by  screening.  Silica 
from  quartz  is  to  be  distinguished  by  the  wedge  shape  of 


PIGMENTS  AND  PAINTS  125 

the  crystals  when  examined  microscopically.  In  this  con- 
dition silica  forms  an  invaluable  base  in  the  preparation  of 
wood  fillers  for  filling  up  the  grain  of  open-grained  woods, 
when  the  crystals  form  with  the  binding  medium  aggregates 
possessing  great  rigidity.  Silica  possesses  little  or  no  opacity 
when  mixed  with  oil,  and  its  oil  absorption  varies  with  its 
fineness  of  division. 

YB1.1.0W  AND  Orange  Pigments 

Lead  Chromates,  Lead  Chromes  or  Chrome  Yellows. 

— ^The  lead  chromates  are  obtained  by  precipitation  of  lead 
acetate  or  nitrate  with  potassium  bichromate.  A  great  variety 
of  tints  ranging  from  a  pale  primrose  to  a  deep  orange-red  can 
be  obtained  by  modifying  the  conditions  of  the  precipita- 
tion. Most  of  the  paler  tints  contain  varying  amounts  of 
lead  sulphate  which  must  not  be  regarded  as  an  adulterant 
but  as  a  necessary  adjunct  in  obtaining  the  light  tone.  The 
preparation  of  the  different  tones  of  chromes:  primrose 
chrome,  lemon,  middle  and  orange  chromes,  and  Derby, 
Persian,  and  Chinese  Reds  {q.v.)  is  somewhat  complicated 
and  carefully  guarded  as  a  trade  secret  by  the  manufacturers. 
In  general,  it  may  be  stated  that  the  redder  tones  of  chromes 
are  obtained  by  increasing  the  basicity  of  the  product. 

The  yellow  and  orange  chromes  are  the  most  important 
of  the  yellow  pigments,  being  possessed  of  a  great  degree 
of  opacity  and  staining  power.  They  are  very  fast  to  light, 
and  fairly  stable  in  acid  fumes,  but  in  common  with  all  lead 
pigments  are  blackened  by  sulphuretted  hydrogen.  They 
should  therefore  be  used  with  caution  in  admixture  with 
sulphur-containing  pigments  such  as  ultramarine.  They 
are  not  as  a  rule  employed  in  distemper  paints  on  account 
of  the  tendency  of  the  free  alkali  to  change  their  tone  to  a 
reddish  hue.  The  lead  chromes  are  used  almost  exclusively 
for  the  manufacture  of  green  pigments  in  combination  with 
Prussian  blue. 

Barium  Chromate  or  BariumChrome. — ^Thispigment, 
sometimes  known  as  lemon  chrome,  is  little  used  nowadays 


126    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


on  account  of  its  low  opacity  and  poor  staining  power.  It 
has  the  advantage,  however,  over  lead  chromes  in  its  stability 
to  sulphuretted  hydrogen. 

Zinc  Chromate  or  Zinc  Chrome. — This  pigment  is 
prepared  by  precipitation  of  zinc  sulphate  with  potassium 
bichromate.  It  has  a  pale  greenish-yellow  colour  and 
possesses  relatively  little  opacity  and  a  lower  staining  power 
than  the  lead  chromes.  It  is  valuable,  however,  in  forming 
a  green  of  particular  beauty  in  combination  with  Prussian 
blue,  the  distinction  from  lead  chrome  greens  being  probably 
based  on  its  relatively  higher  staining  power  than  trans- 
parency. 

Yellow  Pigments  from  Coal-Tar  Dyes. — ^These  have 
little  importance  at  the  present  day  in  the  paint  industry 
on  account  of  their  poor  opacity.  They  are  derived  from 
mono-azo  dyes,  of  which  Hansa  yellow  is  a  well-known  type. 
On  account  of  their  stability  to  alkali  they  find  some 
application  in  distempers. 

The  Yellow  Earth  Pigments  or  Ochres. — ^The  yellow 
ochres  owe  their  colour  to  the  presence  of  hydrated  iron  oxide, 
which  is  present  in  the  earth  in  conjunction  with  silica, 
alumina,  lime,  small  amounts  of  manganese,  and  carbonic 
and  sulphuric  acids.  Their  tone,  opacity,  and  staining 
powers  vary  considerably  in  a  manner  usually  independent 
of  their  composition.  The  colour  varies  from  a  bright 
yellow  in  the  case  of  ochres  mixed  in  Oxfordshire  and 
France  to  a  reddish  yellow  in  the  case  of  those  from  Derby- 
shire, Cumberland,  and  Peru.  The  process  of  manufacture 
consists  of  crushing,  levigation,  and  drying. 

The  palest  varieties  are  found  in  France,  where  they  are 
treated  in  that  country  and  arrive  on  the  market  with 
alphabetical  titles  supposedly  designating  their  colouring 
properties,  e.g.  JF,  Jaune  fin,  JC  Jaune  clair,  JFLS  Jaune 
fin  lesse  surfin,  etc.  They  possess,  as  a  rule  however,  little 
opacity. 

The  West  of  England  mines  produce  a  large  amount  of 
ochres  of  great  variety,  the  characteristic  of  which,  however, 
is  their  ccmparativel}'  poor  puritj^  of  colour,  reduction  with 


PIGMENTS  AND  PAINTS  127 


white  pigments  showing  up  a  dirty  reddish  or  brown  cast. 
The  most  valuable  staining  ochres  come  from  the  neighbour- 
hood of  Siena  in  Italy,  from  which  certain  varieties  are 
exported  which  yield  tints  resembling  yellow  ochres  on 
reduction. 

The  Irish  and  South  American  ochres  are  characterized 
by  high  staining  powers  of  somewhat  dull  colour,  in  addition 
to  which  their  high  content  in  manganese  (0*5  to  4*0  per  cent.) 
render  their  use  in  high-class  paints  and  enamels  somewhat 
dangerous  on  account  of  the  progressive  oxidizing  effect 
conferred  on  the  medium. 

Red  Pigments 

Mercuric  Sulphide  or  Genuine  Vermilion, — ^These 
pigments  are  classified  according  to  their  method  of  prepara- 
tion as  English,  French,  and  Chinese  vermilions,  but  their 
ultimate  composition  is  not  materially  affected  by  their 
method  of  preparation,  although  their  physical  properties 
of  colour,  etc.,  are  thus  influenced.  The  common  method  of 
preparation  is  by  combination  of  metallic  mercury  and 
sulphur  by  heat  to  form  the  black  sulphide  which  is  then 
sublimed  as  the  well-known  red  pigment.  Vermilion  finds 
little  application  nowadays  on  account  of  its  expense, 
toxic  properties,  and  tendency  to  settle  out  in  paint  (sp.  gr. 
8*2).  It  is  fairly  permanent,  and  possesses  good  body  and 
staining  power.  Its  use  is  practically  restricted  to  certain 
special  purposes  where  brightness  and  opacity  are  desired, 
the  modern  lake  pigments  having  almost  entirely  super- 
seded it. 

The  Red  Lake  Pigments. — The  only  naturally-occur- 
ring bright  red  pigment  being  vermilion,  recourse  has  been 
had  to  pigments  obtained  by  precipitation  of  an  organic 
dye  on  an  inorganic  base.  The  dyes  employed  for  this 
purpose  may  be  classified  into  two  varieties  :  the  naturally- 
occurring  dyes,  e.g.  logwood,  cochineal,  and  those  derived 
from  coal-tar  products,  e.g.  the  so-called  coal-tar  dyes. 

(a)  Red    Pigments    from    Natural   Dyestuffs.  These 


128    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


pigments  are  greatly  diminishing  in  importance  of  late  years 
owing  to  their  cost  in  comparison  with  those  obtained  from 
synthetic  colouring  matters.  The  only  colouring  matters  of 
any  importance  used  are  those  of  madder  and  cochineal, 
which  latter  is  used  to  make  carmine,  carmine  lake,  and 
crimson  lake.  Naturally-occurring  madder  has  now  been 
almost  entirely  superseded  by  synthetic  alizarine,  whilst, 
with  the  exception  of  carmine,  which  on  account  of  its  cost 
is  restricted  entirely  to  use  as  an  artist's  pigment,  the  other 
lakes  prepared  by  precipitation  of  cochineal  are  too  fugitive 
to  be  of  any  value  in  paints. 

(&)  Red  Pigments  from  Coal-Tar  Dyes.  These  pigments 
are  produced  by  the  precipitation  or  striking  of  a  water- 
soluble  dye  on  a  base  consisting  of  blanc  fixe,  white  lead, 
lead  sulphate,  orange  lead,  etc.,  by  means  of  a  mordant. 
The  lake  thus  formed  is  thus  analogous  to  a  dyed  fabric. 
The  lakes  derived  from  mono-azo  dyes  are  the  more  numerous 
and  important  from  the  point  of  view  of  pigments.  These 
dyes  consist  of  the  coupling  product  obtained  by  com- 
bination of  a  diazotized  amine  with  a  phenol,  naphthol, 
amine,  or  derivatives  of  the  same.  Two  of  the  most 
important  dyes  used  for  the  preparation  of  bright  red  lake 
pigments  are  Ivithol  Red  B  and  Paranitraniline  Red,  which 
are  respectively  sulphonaphthaleneazo-/8-naphthol,  and  p- 
nitrobenzene-azo-^-naphthol.  The  former  forms  perhaps  the 
most  important  red  lake  pigment  used  in  the  paint  trade, 
and  is  characterized  by  a  great  degree  of  fastness  and 
insolubility  in  oil  (''non-bleeding'').  The  paranitraniline 
reds,  although  very  fast  to  light  and  of  great  brilliance,  are 
liable  to  bleeding."  Many  other  important  pigments  are 
obtained  from  dyes,  of  which  the  naphthol  sulphonic  acids 
are  the  starting  points.  Helio  Fast  Red  RIy  Toluidine 
toner  ")  a  derivative  of  m-nitroparatoluidine-i:3:4,  is  also 
an  important  lake-producing  dyestuff,  and  is  one  of  the 
fastest  to  light.  (For  an  account  of  the  mono-azo  dyestuffs 
used  in  the  manufacture  of  pigments,  see  AUsebrook,  Proc, 
Oil  and  Colour  Chem,  Assoc.,  1919,  2,  14.) 

Madder,  or  alizarine  lake,  is  obtained  by  combination  of 


PIGMENTS  AND  PAINTS 


129 


alizarine  dyestuff  with  aluminium  hydrate.  It  forms  a 
deep-red  somewhat  transparent  pigment  with  a  fine  blue 
undertone,  and  is  useful  in  obtaining  shades  of  maroon  and 
rich  purples.  It  is  extremely  fast  to  light,  but  is  apt  to 
retard  drying  of  the  medium. 

The  eosine  lakes,  or  vermilionettes,  are  prepared  by  pre- 
cipitation of  derivatives  of  fluorescein  with  lead  or  aluminium 
salts.  Although  of  very  bright  shade  they  are  very  fugitive 
to  light,  and  it  is  the  practice  to  minimize  their  fugitiveness 
by  precipitating  the  lakes  on  lead  bases,  e,g,  white  lead, 
lead  sulphate,  or  orange  lead.  They  are  gradually  being 
replaced  by  the  far  more  permanent  naphthylamine  reds. 

Derby  Red,  Persian  Red,  Chinese  Red,  or  American 
Vermilion. — ^This  pigment  consists  essentially  of  basic  lead 
chromate,  the  details  of  preparation  of  which,  however, 
are  jealously  guarded  as  secrets  by  manufacturers.  Their 
properties  resemble  those  of  the  lead  chromes,  but  their 
red  colour  is  dependent  on  their  crystalline  form,  since 
they  readily  revert  to  orange  or  yellow  pigments  by  loss 
of  their  crystalline  structure  on  grinding.  This  latter 
disadvantage,  coupled  with  their  coarse  structure  and 
somewhat  dull  colour,  results  in  their  comparative  unim- 
portance as  red  pigments.  American  vermilion,  as  it  is 
termed  in  the  United  States,  enjoys  a  certain  popularity 
as  a  rust-inhibitive  pigment  in  that  country. 

The  Iron  Oxides  Reds. — ^Many  of  these  are  naturally- 
occurring,  but  with  the  exception  of  the  pigment  known  as 
red  ochre,  they  are  also  obtained  artificially  by  calcination 
of  yellow  ochres,  waste  iron  liquors,  ferrous  sulphate,  etc., 
whilst  bearing  a  similar  appellation  irrespective  of  their 
method  of  preparation. 

Of  the  iron  oxides,  red  ochre  possesses  but  little  import- 
ance. It  is  a  red  oxide  of  iron  with  a  Fe203  content  of 
about  10  per  cent.,  and  small  quantities  have  been  mined 
in  Yorkshire. 

Indian  Red  was  the  name  originally  given  to  a  very  pure 
form  of  red  iron  oxide  found  in  India,  but  the  term  is  now 
applied  to  iron  oxides  produced  by  calcination  of  iron  liquors, 
s.  9 


130    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Three  varieties  of  Indian  Red  are  recognized  in  the  trade, 
known  as  light,  middle,  and  deep.  The  Fe203  content 
varies  from  about  50-95  per  cent.  The  Indian  reds  are 
characterized  by  a  very  high  degree  of  opacity  and  staining 
power,  and  in  consequence  of  their  careful  preparation,  are 
finely  divided  pigments  employed  in  high-class  practice. 

Spanish  red  oxide  is  a  naturally-occurring  iron  oxide 
mined  in  Spain.  Being  of  rather  coarse  texture,  and  of 
fairly  low  iron  oxide  content  (20-40  per  cent.),  its  use  is 
restricted  to  coarse  red  oxide  paints  for  the  protection  of 
metal  work  and  generally  where  fineness  is  not  a  primary 
consideration,  such  as  for  backing  "  in  linoleum  manu- 
facture. It  is  considered  by  many  authorities  to  be  a 
powerful  inhibitor  of  rust. 

Red  oxide  of  iron  is  a  generic  term  covering  the  classes 
of  iron  oxide  of  medium  Fe203  content,  whether  of  purely 
natural  origin  or  obtained  by  calcination  of  yellow  ochres. 

Venetian  Red  is  a  name  sometimes  given  to  the  above- 
described  red  iron  oxide,  but  generally  speaking,  the  term 
is  usually  applied  to  artificially-prepared  iron  oxides  pro- 
duced by  heating  precipitates  obtained  by  the  interaction 
of  ferrous  sulphate  with  slaked  lime.  They  consist,  therefore, 
of  mixtures  of  ferric  oxide  and  calcium  sulphate,  to  which 
barytes  is  sometimes  added  to  still  further  reduce  the  cost. 
Venetian  Red  has  a  brighter  colour  than  light  Indian  Red, 
and  possesses  a  staining  power  and  opacity  roughly  pro- 
portional to  its  iron  oxide  content. 

Turkey  Red,  scarlet  red,  rouge,  and  colcothar  are  the 
purest  form  of  bright  oxides  of  iron  obtained  by  calcination 
of  iron  liquors.  A  few  varieties  of  naturally-occurring 
oxides  found  in  the  West  of  England  pass  under  the  designa- 
tion of  Turkey  Red.  Turkey  Red  usually  contains  from 
95-99  per  cent,  of  ferric  oxide,  and  possesses  a  very  high 
degree  of  both  opacity  and  staining  power.  These  reds 
fetch  the  highest  prices  ruling  on  the  market  for  iron  oxide 
reds,  and  their  use  is,  as  a  rule,  restricted  to  purposes  where 
very  high  opacity  is  needed  in  a  paint  or  enamel  or  for  use  as 
stainers.    In  conjunction  with  madder  lake  very  rich  deep 


PIGMENTS  AND  PAINTS 


131 


reds  are  obtained,  the  high  opacity  of  the  iron  oxide  red 
compensating  for  the  transparency  of  the  lake. 

The  red  oxides  are  exceedingly  permanent  to  light  and 
most  chemical  agencies.  When  the  artificial  varieties  of  the 
oxides  are  carefully  prepared,  i.e.  free  from  either  lime 
originating  from  the  precipitant  or  sulphuric  acid  left  as  a 
residue  from  decomposition  of  the  sulphate,  they  are  very 
inactive  to  the  medium.  The  presence  of  traces  even  of 
sulphuric  acid,  however,  is  very  deleterious,  and  often  explains 
the  retardation  of  drying  which  occurs  in  paints  containing 
them. 

Orange  Lead. — ^Although  rarely  used  directly  as  a  pig- 
ment this  product  is  one  of  the  most  important  bases  used 
for  the  precipitation  of  dye  lakes.  It  is  obtained  by  roasting 
massicot,  the  oxide  of  lead  obtained  by  decomposition  of 
white  lead,  in  suitably  constructed  chambers  with  access  of 
air.  In  addition  to  its  slightly  more  orange  tint,  it  differs 
from  red  lead  in  its  higher  content  of  Pb02,  which  is  obtained 
by  similarly  oxidizing  litharge,  the  fused  variety  of  lead 
oxide  prepared  directly  from  the  metal.  This  factor  is 
indirectly  of  importance  in  correspondingly  reducing  the 
proportion  of  free  PbO,  an  undesirable  constituent  on 
account  of  its  basicity  and  consequent  reactivity  with  acid 
media. 

Red  lead  is  but  rarely  used  as  a  pigment  alone,  its  main 
application  being  that  of  an  addition  to  white  lead  or 
priming  paints  for  reasons  which  are  not  very  apparent,  but 
are  most  probably  the  result  of  custom. 

Brown  Pigments 

Raw  Sienna  is  a  name  given  to  a  class  of  earth  pigments 
of  brownish-yellow  colour.  The  original  pigment  was  found 
near  Siena  in  Italy,  but  the  pigment  is  also  mined  in 
Devonshire,  Cumberland,  and  in  America.  Sienna  resembles 
yellow  ochre  in  composition,  but  does  not  possess  quite 
the  opacity,  whilst  its  staining  power  is  higher.  Its  use, 
therefore,  is  restricted  to  that  of  a  stainer. 


132    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 

Burnt  Sienna. — Burnt  sienna,  as  its  name  implies,  is 
obtained  by  calcination  of  the  raw  variety.  It  has  an 
orange-brown  colour  and  in  its  staining  strength  resembles 
raw  sienna.    Both  varieties  are  quite  fast  to  light. 

Raw  and  Burnt  Umbers. — ^These  are  distinguished  as 
Turkey  umbers  and  English  umbers,  the  former  being 
mined  in  Cyprus,  and  being  the  most  valued  on  account  of 
its  strength  and  purity  of  colour.  Raw  umber  varies  in 
colour  from  a  yellowish-brown  to  a  dark-brown,  the  burnt 
variety  (obtained  by  calcination)  having  a  somewhat  warmer 
tone.  In  composition  they  resemble  highly  manganiferous 
ochres,  the  manganese  content  reaching  as  high  as  20  per 
cent,  in  some  cases.  The  umbers  are  of  good  opacity  and 
high  staining  power,  and  are  quite  permanent  to  light. 
On  account,  however,  of  their  high  manganese  content  they 
must  be  used  with  caution.  Crude  unground  umber  is  used 
by  the  oil  boiler  as  a  drier  for  dark-coloured  oils,  both  iron 
and  manganese  going  into  solution  in  the  oil. 

Vandyke  Brown,  Cassel  Earth,  and  Cappagh  Brown. 
— ^These  pigments  are  naturally-occurring  earths  found  in 
Germany  and  Ireland.  In  composition  they  resemble  umber, 
but  contain  in  addition  a  bituminous  substance  from  which 
they  probably  derive  their  colour.  Vandyke  brown  is  not 
very  fast  to  light,  and  is  so  transparent  that  its  use  is  entirely 
restricted  to  that  of  a  stainer,  in  which  respect  it  finds 
considerable  use  when  ground  with  either  water  or  oil  for 
conferring  a  walnut  colour  to  wood. 

PURPI.E  Pigments 

Apart  from  the  lakes  obtained  from  purple  dyes,  for  the 
description  of  which  the  reader  is  referred  to  the  larger 
text-books,  an  oxide  of  iron  of  brownish-purple  tint  is  found 
on  the  market.  This  is  known  as  Purple  Brown  or  Purple 
Oxide  of  Iron.  It  is  usually  obtained  by  calcination  of  iron 
liquors,  and  when  thus  made  does  not  differ  markedly  from 
deep  Indian  red.  It  occurs  naturally  in  the  West  of  England 
mines. 


PIGMENTS  AND  PAINTS 


133 


Bi,uE  Pigments 

Prussian  Blue. — series  of  pigments  prepared  by  inter- 
action of  alkali  ferrocyanides  and  ferricyanides  with  ferrous 
or  ferric  iron  salts  are  of  great  importance  in  the  paint  trade. 
The  different  varieties,  which  differ  in  their  shades  accordmg 
to  the  materials  used  and  different  conditions  of  preparation, 
come  on  to  the  market  under  various  names.*  Although 
the  generic  name  of  Prussian  blue  serves  to  distinguish 
pigments  composed  of  basic  iron  in  combination  with  the 
acid  radicles  ferrocyanogen  and  ferricyanogen,  the  name  is 
often  applied  to  a  particular  shade  and  strength  of  pigment. 
Chinese  blue  or  bronze  blue  is  considered  the  finest  of  these 
colours,  and  is  distinguished  by  its  bronze  cast,  which 
causes  it  to  find  much  favour  in  the  manufacture  of  printing 
ink.  Prussian  blue  is  a  second  grade  of  Chinese  blue.  The 
names  Paris  blue,  Antwerp  blue,  Berlin  blue,  Milori  blue 
represent  varieties  slightly  differing  in  shade  from  Prussian 
blue,  and  are  names  but  little  used  in  the  paint  trade. 
Brunswick  blue  is  a  reduced  Prussian  blue,  i.e.  a  mixture 
of  Prussian  blue  and  barytes. 

The  Prussian  blues  are  pigments  of  a  very  high  degree 
of  staining  power,  but  possessed  of  little  opacity.  They 
are  fairly  fast  to  light,  but  have  the  curious  property  of 
bleaching  on  exposure  to  strong  light  and  regaining  their 
colour  in  the  dark.  They  are  resistant  to  acid  fumes,  but 
are  decomposed  readily  by  alkalies,  changing  to  the  reddish- 
brown  colour  of  ferric  hydroxide.  Care  must  therefore  be 
exercised  that  they  are  not  used  in  combination  with  pig- 
ments liable  to  contain  alkali  material,  such  as  whiting  or 
China  clay.  Prussian  blues  are  therefore  inadmissible  as 
pigments  in  distempers. 

*  Although  Prussian  blues  were  formerly  produced  nearly  exclusively 
from  potassium  ferrocyanide  or  ferricyanide,  the  difficulty  of  obtaining 
potash  during  the  war  led  to  the  preparation  of  these  blue  pigments 
from  the  iron-cyanogen  compounds  of  sodium.  Since  a  small  but  definite 
proportion  of  adsorbed  alkali  salt  forms  an  integral  part  of  Prussian  blues, 
the  substitution  of  sodium  for  potassium  is  not  without  effect  on  the  hue 
of  the  resulting  pigment,  the  sodium  blues  falHng  behind  those  from 
the  potassium  salt  in  the  beauty  of  their  tones. 


134    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Ultramarine  Blue. — Originally  this  pigment  was  ob- 
tained from  its  natural  source  of  lapis  lazuli,  and  is  stated  to 
be  still  obtained  from  this  mineral  for  use  as  an  artist's  pig- 
ment. It  is  a  very  important  pigment,  and  is  manufactured 
artificially  in  very  large  quantities  by  roasting  a  mixture  of 
silica,  China  clay,  sulphur,  sodium  carbonate,  and  sulphate. 
Its  ultimate  constitution  is  not  known,  and  the  different 
varieties  which  vary  considerably  in  their  tone  differ  in  their 
chemical  composition. 

Ultramarine  is  a  pigment  of  exceedingly  bright  blue 
colour  which  varies  from  a  greenish-blue  to  a  deep  bluish 
violet.  It  does  not  possess  great  opacity,  and  is  not  so 
strong  in  staining  as  Prussian  blue.  It  is  very  fast  to  light 
and  alkalies,  but  is  immediately  decomposed  on  contact 
with  acids  with  discharge  of  the  colour  and  liberation  of 
sulphuretted  hydrogen.  Ultramarine  is  liable  to  contain 
small  quantities  of  free  sulphur,  and  on  this  account  must 
not  be  used  in  combination  with  lead  pigments  which  give 
a  black  sulphide.  Ultramarine  is  occasionally  liable  to 
retard  drying,  probably  on  account  of  its  free  sulphur. 

Lime  Blue  is  an  inferior  variety  of  ultramarine  often 
reduced  with  China  clay,  which  is  used  as  the  principal 
blue  pigment  in  distemper.  The  name  was  originally 
applied  to  pigments  prepared  by  precipitating  basic  copper 
with  lime,  but  which,  however,  are  now  obsolete. 

Green  Pigments 

Brunswick  Greens,  of  which  several  tints  are  made,  and 
sometimes  designated  as  light,  middle,  deep,  and  extra  deep, 
consist  of  a  mixture  of  Prussian  blue  and  chrome  yellow. 
The  cheaper  varieties  are  made  by  reducing  the  above  with 
barytes.  The  chrome  yellow  (lead  chromate) ,  when  replaced 
by  the  more  transparent  zinc  chromate,  yields  a  green  of 
peculiar  beauty,  which,  when  combined  with  yellow  ochres, 
or  umber,  yield  a  series  of  fine  olive  greens.  The  Brunswick 
greens  are  pigments  of  high  staining  power  and  good  opacity, 
combining  the  properties  of  their  two  constituents. 


PIGMENTS  AND  PAINTS 


Chrome  Green  or  Guignet's  Green. — ^This  is  a 
hydrated  chromium  oxide  prepared  by  heating  together 
potassium  bichromate  and  boric  acid.  It  finds  its  principal 
appHcation  as  a  pigment  for  indiarubber  as  it  withstands  the 
temperature  of  vulcanization  and  the  action  of  sulphur  with- 
out losing  its  colour.  It  is  very  fast  to  light  and  chemical 
agencies,  but  is  of  too  poor  a  body  and  too  dull  in  colour 
to  commend  itself  for  use  as  a  paint  pigment. 

Emerald  Green. — ^This  pigment  consists  of  aceto-arsenite 
of  copper,  and  on  account  of  its  poisonous  properties  is 
practically  obsolete  as  a  painter's  pigment.  Its  principal 
use  is  as  an  insecticide.  It  is  characterized  by  a  very  high 
degree  of  permanence  and  high  opacity,  whilst  its  bright  hue 
is  unequalled  by  any  other  pigment. 

The  Green  Lakes  have  not  found  any  extensive  appli- 
cation in  the  paint  industry  on  account  of  the  superiority  of 
the  Brunswick  greens.  I^akes  from  malachite  green,  naph- 
thol  green,  etc.,  struck  on  green  earth  (see  below)  are  used 
in  distempers  on  account  of  the  non-resistance  to  alkali  of 
the  constituent  Prussian  blue  in  Brunswick  green. 

Green  Earth  is  a  naturally-occurring  earth  consisting  of  a 
hydrated  silicate  of  magnesium  and  aluminium  and  containing 
small  amounts  of  iron.  The  bulk  of  it  is  mined  in  Germany, 
but  a  certain  amount  is  found  in  this  country.  It  has  a 
dull  green  colour,  and  has  little  opacity  or  staining  power. 
It  is  very  fine  in  texture  and  forms  a  good  adsorbent  or  base 
for  basic  dyes  which  can  be  precipitated  thereon  without  a 
mordant. 

BivACK  Pigments 

The  black  pigments  used  by  the  painter  are,  in  all  cases 
but  one,  varieties  of  carbon  black  obtained  by  burning 
organic  substances  in  an  atmosphere  deficient  in  oxygen. 

Black  Oxide  of  Iron. — Black  oxide  of  iron,  Fe304,  is  a 
naturally-occurring  pigment  found  in  ochre  deposits  in 
England.  Whilst  it  possesses  neither  the  transparency  nor 
the  staining  power  of  the  carbon  blacks,  it  finds  some 


136    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


application  in  paints  on  account  of  its  comparatively  low 
oil  adsorption  (about  30  per  cent.). 

Gas  Black  consists  of  a  crude  form  of  carbon  obtained 
as  a  by-product  in  gas  works.  It  is  the  poorest  of  the  carbon 
black  pigments  and  finds  but  little  use  in  paints.  Its  main 
application  is  in  colouring  mortar  and  cement. 

Lamp  Black  and  Vegetable  Black. — ^Lamp  black  is 
obtained  by  the  incomplete  combustion  of  various  oils  in 
specially-constructed  chambers.  It  is  graded  into  high  and 
low  qualities  by  the  distance  at  which  the  particles  are 
allowed  to  deposit  from  the  combustion  chamber.  The 
finest  variety,  known  as  vegetable  black,  is  relatively  free 
from  adhering  decomposition  products  of  the  oil  used  which 
would  exercise  a  retarding  action  of  the  paint  into  the  com- 
position of  which  it  enters.  Lamp  black  is  a  very  permanent 
pigment  of  high  opacity  and  staining  power.  On  reduction 
with  white  pigments  it  gives  cold  greys. 

Drop  Black  or  Frankfort  Black. — Drop  black,  so-called 
from  the  practice,  now  becoming  obsolete,  of  placing  it  on 
the  market  in  tear-shaped  masses  agglomerated  by  means 
of  a  weak  solution  of  size,  is  obtained  by  heating  twigs  of 
trees,  etc.,  in  a  closed  vessel  and  subsequently  washing  and 
drying  the  carbonized  residue.  It  is  a  black  of  very  intense 
colour,  but  of  a  lower  degree  of  strength  than  the  other 
varieties  of  carbon  blacks.  It  is  rapidly  being  superseded 
by  vegetable  black  and  carbon  black. 

Ivory  Black  and  Bone  Black. — As  its  name  implies, 
ivory  black  is  obtained  by  heating  ivory  waste  in  closed 
vessels  in  the  same  manner  as  drop  black.  Bone  black  is  the 
carbonaceous  residue  from  the  distillation  of  bones  in  the 
manufacture  of  bone  oil  or  Dippel's  oil,  but  in  many  cases 
the  term  ivory  black  is  used  indiscriminately  for  both 
varieties.  Ivory  black  is  a  black  of  very  intense  colour,  and 
is  chiefly  prized  on  this  account,  being  little  used  as  a 
staining  pigment. 

Carbon  Black. — Carbon  black  is  formed  by  the  ignition 
of  American  natural  gas  from  the  sources  in  the  neighbour- 
hood of  Pittsburg,  under  suitable  conditions  of  limitation  of 


PIGMENTS  AND  PAINTS  137 


access  of  air.  It  is  the  purest  form  of  carbon  pigment 
known,  averaging  from  95-99  per  cent,  purity.  It  possesses 
exceptionally  high  staining  power  and  opacity,  and  its  use 
is  becoming  increasingly  extensive.  It  differs  from  lamp 
black  in  giving  warm  browns  on  reduction  with  white  pig- 
ments. It  is  of  exceedingly  fine  texture,  and  works  well  either 
in  oil  or  water,  being  devoid  of  either  oily  or  aqueous  impurity. 
It  is  used  for  the  manufacture  of  high-class  black  enamels, 
where  its  fineness  of  texture  renders  it  especially  valuable. 
(Perrott  and  Thiessen,  /.  Ind.  Eng.  Ghent.,  1920,  12,  324.) 

Paints,  Enambi^s,  and  Distempers  (Water-Paints) 

There  exists  no  hard-and-fast  distinction  between  the 
three  classes  of  products  enumerated  above,  paints  and 
enamels  especially  being  terms  somewhat  indiscriminately 
used  in  many  cases  for  a  similar  class  of  product.  Distemper 
or  water-paint,  also  known  sometimes  by  the  American 
appellation  of  Kalsomine,''  differs  rather  more  in  its 
composition,  in  that  the  bulk,  or  sometimes  indeed  the  whole 
of  the  fluid  portion  of  the  contained  medium  consists  of 
water.  The  three  classes  of  product,  however,  are  char- 
acterized by  the  fact  that  the}^  are  designated  for  use  as  a 
decorative  as  well  as  a  protective  purpose,  distemper  or 
water-paint,  however,  possessing  little  or  no  protective 
action  for  reasons  to  be  entered  into  later.  A  more  strict 
definition  of  the  three  representative  classes  of  products 
intended  for  decorative  coverings,  however,  will  be  given. 

Paint,  or  ready-mixed  paint,"  as  it  is  commonly 
termed,  consists  of  a  suspension  of  a  pigment  or  mixture  of 
pigments  in  a  medium  consisting  of  raw  or  boiled  linseed  or 
other  drying  oil,  the  rate  of  drying  of  which  has  been  acceler- 
ated by  addition  of  a  drier,  the  fluidity  having  been  reduced 
to  a  practicable  consistency  for  application  by  means  of  a 
small  amount  of  turpentine  or  turpentine  substitute.  The 
primary  object  of  the  contained  pigment  in  paint  is  to  confer 
the  desired  colour  to,  or  obscuration  of  the  ground  covered. 
There  are,  however,  certain  other  characteristics  of  paint 


4 


138    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


which  warrant  its  consideration  from  a  standpoint  other  than 
that  of  merely  a  pigmented  Hnseed  oil.  In  the  first  place, 
it  is  a  well-known  fact  that  the  film  obtained  by  allowing 
linseed  oil  to  dry  on  a  surface  hardly  possesses  those  qualities 
of  hardness,  impermeability,  weather- resistance,  etc.,  re- 
quired to  adapt  it  to  serve  as  an  efficient  protective  coating. 
The  porosity  of  dried  linseed  oil  has  been  discussed  in 
another  chapter,  and  the  origin  of  this  porosity  is  most 
probably  to  be  found,  on  the  one  hand,  in  the  intrinsic 
nature  of  the  film  or,  on  the  other  hand,  in  the  expansion 
and  subsequent  contraction  in  volume  which  the  film 
suffers  during  and  subsequent  to  its  setting  to  a  solid. 
The  partial  retention  of  the  volume  of  the  film  is  obtained 
by  the  presence  of  the  pigment  which  acts  similarly 
to  the  inert  matter  which  is  added  to  various  cements. 
Another  point  of  importance  is  to  be  found  in  the  hardening 
action  conferred  by  the  pigment  on  the  film,  acting  as  it 
does  sometimes  by  chemical  action  in  forming  a  heavy 
metal  soap  (lead  linoleate,  zinc  linoleate,  etc.),  or  merely  by 
physical  effect  (silica,  barytes,  etc.) .  Finally,  owing  to  the 
two-phase  solid-liquid  system  created,  effects  of  surface 
viscosity  would  result  in  a  thicker  film  being  applied 
than  would  be  possible  in  the  case  of  the  unpigmented 
medium.  Consideration  of  the  attainment  of  obscuration 
or  decorative  effect  by  the  presence  of  the  pigment  has 
been  touched  upon  in  an  earlier  chapter  and  need  not  be 
entered  into  beyond  that  arising  out  of  the  physico-chemical 
effects  produced  by  absorption  of  certain  rays  by  the  pig- 
ment.* Thus  the  inhibition  of  absorption  of  ultra-violet 
rays  and  degradation  to  waves  of  longer  wave-length  has  been 
obtained  in  coatings  intended  for  aircraft,  by  the  use  of 
aluminium  powder. 

In  actual  practice,  however,  the  attainment  of  the 
necessary  properties  of  a  paint  devolves  almost  entirely  on 
the  pigmentation  of  the  medium  to  a  degree  necessary  to 
obtain  a  reasonable  opacity,  colour,  and  fluidity  (ease  of 
application) .    Thus,  it  is  obvious  that  very  light  pigmentation 

*  Eng.  Pat.  131,641/1918. 


PIGMENTS  AND  PAINTS  139 


with  a  finely-divided  active/'  i.e,  basic  pigment,  such 
as  zinc  oxide,  would  be  sufficient  to  overcome  the  porosity 
due  to  sub-microscopic  pores,  whilst  the  ever-present  slight 
acidity  of  raw  or  boiled  linseed  oil  would  result  in  the  forma- 
tion of  a  metallic  soap  which  would  confer  an  increased 
hardness  to  the  film.  However,  pigmentation  beyond  this 
stage  is  always  attained  in  paints,  and  the  limit  is  bounded 
by  the  point  at  which  excessive  addition  has  the  effect  of 
leaving  the  pigment  in  an  insufficiently  bound  state.  This 
latter  condition  may  be  one  of  several  degrees  ;  the  first 
stage  of  which  would  be  represented  by  the  condition  in 
which  the  paint  film  would  dry  with  a  surface  devoid  of 
gloss  flat  paint''),  but  with  a  considerable  degree  of 
hardness  and  resistance  to  wear.  Pigmentation  to  a  further 
degree,  however,  would  ultimately  result  in  the  dried  film 
being  in  a  powdery  condition  due  to  insufficient  cementing 
medium.  Beyond  the  degree  of  pigmentation  above  referred 
to  as  necessary  to  fill  the  sub-microscopic  pores  of  the  linseed 
oil  film,  further  introduction  of  pigment  results  in  diminished 
impermeability  to  gases  and  moisture.  This  point  is  borne 
out  by  the  results  of  some  unpublished  experiments  by  one 
of  the  authors.  When  paint  films  of  different  degrees  of 
pigmentation  were  examined  for  their  permeability  to  water 
vapour,  it  was  found  that,  beyond  the  point  at  which  drying 
resulted  in  a  matt  coating  being  obtained,  a  very  great  degree 
of  permeability  resulted,  intermediate  degrees  of  permeability 
being  roughly  proportional  to  increasing  pigmentation.* 

In  practice  it  is  found  that  the  degree  of  pigmentation 
necessary  in  white  paints  is  arrived  at  by  obtaining  that 
balance  of  pigment  to  medium  when  a  maximum  opacity  is 
produced  without  impairing  more  than  is  necessary  the 
protective  action  of  the  film,  or,  in  other  words,  its  impermea- 
bility. This  latter  point  is  judged  by  the  relative  gloss  on 
the  dried  film  since  it  is  obvious  that  high  pigmentation 
reduces  the  gloss  of  the  film. 

At  this  stage  it  is  necessary  to  introduce  the  subject 

*  See  also  A.  de  Waele,  Proc.  Oil  and  Colour  Chemists*  Association, 
99,  2,  13,  106. 


140    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


of  a  qualifying  factor  when  referring  to  relative  pigmenta- 
tion/' In  the  chapter  on  Pigments,  the  meaning  of  the 
term  oil  absorption  has  been  explained  as  that  minimum 
proportion  of  oil  necessary  to  transform  unit  weight  of  dry 
pigment  to  a  definite  paste.  Taking  as  a  specific  instance 
two  pigments  of  extreme  oil  absorptions — white  lead  and 
carbon  black,  having  oil  absorptions  of  8  per  cent,  and  about 
250  per  cent,  respectively — it  is  obvious  that  the  relative 
pigmentation  of  two  paints  containing,  in  the  one  case, 
white  lead,  and  in  the  other,  carbon  black,  will  to  a  certain 
extent  be  dependent  on  this  factor  of  oil  absorption.  Thus, 
a  white  lead  ready-mixed  paint  containing  30  lbs.  of 
raw  linseed  oil  per  100  lbs.  of  pigment  will  be  relatively  less 
pigmented,  ix,  better  bound  with  medium  than  a  carbon 
black  paint  having  the  same  relative  proportions  of  pigment 
to  medium.  A  carbon  black  paint  of  relatively  similar 
pigmentation  to  the  white  lead  paint  of  the  composition 
described  would  therefore  contain  considerably  more  oil 
although  the  relationship  is  not  directly  proportional  to  the 
relative  oil  absorptions  of  the  two  pigments.* 

The  question  of  the  application  of  the  two  systems  of 
uses  of  pigmented  coatings,  under  their  respective  designa- 
tions of  paints  and  enamels,  will  be  dealt  with  next,  in 
order  to  show  the  differentiation  between  the  two  products. 
The  classification  into  paints,  on  the  one  hand,  and  enamels 
on  the  other,  is  more  easily  grasped  when  the  two  classes 
of  products  are  regarded  more  as  materials  employed  in 
different  systems  of  protective  decoration  than  merely  as 
different  products.  Considering  first  of  all  the  problem  of 
the  protection  of  a  surface,  preferably  wood,  the  various 
factors  arising  in  the  finished  coating  are  : — 

(i)  Obscuration  of  underground  and  pigmentation. 

(ii)  The  application  of  a  weather-proof  and  wear-proof 

layer  to  isolate  the  underground  from  the  destruc- 
tive influence  of  weather,  abrasion,  etc. 

(iii)  The  production  of  a  final  layer  on  the  surface  of 

such  a  finish  as  to  hide  the  physical  imperfections 

*  de  Waele  (loc.  cit.),  p.  115. 


PIGMENTS  AND  PAINTS  141 


of  the  underground,  e.g,  holes,  cracks,  grain  of 
wood,  etc. 

In  the  system  of  use  of  paints,  all  three  objects  are 
obtained  simultaneously  as  far  as  the  perfection  of  the 
product  renders  it  possible,  by  repeated  applications  of 
layers  of  paint  of  nearly  similar  composition,  obscuration 
being  dependent  on  the  use  of  a  sufficient  number  of  layers. 
The  weather  resistance  and  resistance  to  mechanical  wear 
obtainable  is,  however,  limited  by  those  properties  intrinsi- 
cally found  in  the  medium  used,  both  raw  and  boiled  linseed 
oil,  however,  not  being  very  efficient  in  this  respect.  The 
third  factor  of  finish,''  depending  as  it  does  on  the  final 
attainment  of  a  plane,  porcelain-like  surface,  whether  glossy 
or  otherwise,  is  impossible  of  realization  in  a  paint  of  the 
composition  referred  to,  for  reasons  which  will  be  entered 
into  when  discussing  enamels.  It  is  necessary  to  remark, 
when  considering  the  system  of  application  of  paint,  that 
although  no  actual  differences  are  usually  made  in  the 
composition  of  the  materials  used  in  the  different  coatings, 
in  the  initial  or  priming  coat  a  paint  of  somewhat  different 
physical  properties  is  required.  The  reason  becomes  apparent 
when  the  first  coating  is  applied  to  a  very  absorbent 
surface,  e.g,  wood.  Owing  to  the  capillarity  of  the  wood 
cells  it  is  necessary  to  apply  a  coating  which  will  not 
only  penetrate  the  grain  of  the  wood  to  form  a  key  "  for 
the  next  coating,  but  that  this  coating,  once  dried,  shall 
not  be  so  reduced  in  its  binder  ''  or  oil  medium  by  capillary 
attraction  of  the  wood  cells  that  it  will  powder  off.  This 
end  is  attained  by  pigmenting  the  paint  somewhat  more 
lightly  in  order  to  ensure  more  available  medium  whilst 
reducing  the  viscosity,  or,  in  other  words,  increasing  the 
penetrative  power  by  diluting  the  oil  medium  more  largely 
with  volatile  thinner  (turpentine  or  turpentine  substitute). 
A  proportion  of  red  lead  along  with  the  white  lead  or  other 
pigment  chosen  is  usually  added,  with  the  alleged  object 
of  promoting  oxidation  of  the  oil  deep  in  the  cell  layers. 

In  the  system  of  painting  by  application  of  enamels, 
however,  the  three  desiderata  required  in  the  protective 


142    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


coating  are  obtained  severally  by  successive  application  of 
layers  of  differing  physical  properties  and  composition. 
The  priming  or  first  layer  as  a  rule  differs  little  in  composi- 
tion from  that  used  in  ordinary  paints,  although  it  must 
be  stated  that  since  enamels  are  usually  sold  as  proprietary 
preparations  of  the  different  manufacturers  and  their  com- 
positions are  not  divulged,  they  allow  of  more  ingenuity 
being  exercised  in  their  preparation  with  a  view  to  the 
attainment  of  a  more  efficient  sealing  or  priming  "  coating. 
Thus,  in  the  first  place,  it  will  readily  be  conceded  that  the 
necessity  for  obscuration  to  a  degree  beyond  that  necessary 
for  convenience  in  application  in  this  coating  is  hardly 
necessary,  whilst,  on  the  other  hand,  a  quick-setting  film, 
drying  without  a  gloss,  but  leaving  a  surface  with  a  better 
**key''  or  surface  for  subsequent  coating  than  would  be 
afforded  by  amorphous  pigment  particles  is  desirable.  This 
end  is  attained  by  choice  of  a  suitable  medium  and  incorpora- 
tion therewith  of  hard-setting  pigments,  e.g.  white  and  red 
lead,  together  with  pigment  possessing  good  bite or 
key,''  e,g,  silica  from  crushed  quartz.  Improvements  are 
also  obtained  by  incorporating  with  the  product  sub- 
stances which  reduce  the  tendency  of  the  heavier  pig- 
ments to  settle  in  the  container,  e.g.  China  clay,  asbestine, 
etc. 

In  the  subsequent  coats,  a  layer  of  sufficient  thickness 
to  cover  over  any  imperfections  existing  in  the  grain  of  the 
wood  is  required.  Here,  again,  the  necessity  for  obscuration 
or  pigmentation  is  not  paramount,  the  requirements  of  a 
comparatively  quick-drying,  well-flowing  coat  of  reasonable 
thickness  being  of  more  importance.  These  objects  are 
obtained  by  the  use  of  specially-treated  media  with  a  degree 
of  pigmentation  consistent  with  moderate  porosity.  Solidity 
or  hardness  of  the  coating  is  obtained  by  choice  of  suitable 
pigments  usually  without  regard  to  their  colour.  A  good 
solid  foundation  obtained  as  above  is  regarded  in  practice 
as  the  basis  of  decorative  work,  a  levelling-off  to  a' plane 
smooth  surface  being  attained  by  the  practice  of  cutting 
away    surface    imperfections    with    pumice,    etc.,  and 


PIGMENTS  AND  PAINTS 


water,  leaving  a  stone-like  ground  for  the  application  of  the 
succeeding  or  decorative  coats. 

Actual  pigmentation  is  produced  in  the  coatings  next 
considered,  white  being  taken  under  this  heading  for  pur- 
poses of  convenience.  In  this  coating,  neither  weather- 
resistance  nor  impermeability  need  be  considered,  as  these 
requirements  need  only  be  present  in  the  final  or  surface 
layer.  The  pigmenting  or  colour coat  should  consist 
of  a  pigment  or  mixture  of  pigments  of  maximum  strength 
(tinctorial  power  or  opacity)  ground  in  a  medium  which, 
although  not  limited  in  the  proportion  of  drier  it  contains  by 
any  considerations  of  weather-resistance  (excessive  amount 
of  driers  being  detrimental  to  lasting  properties  in  an 
exposed  film),  should,  however,  not  be  siccatized  to  such 
an  extent  that  any  appreciable  solution  in  the  final  or 
protective  layer  occurs.  A  very  fine  degree  of  subdivision 
of  the  pigment  and  drying  of  the  coat  to  a  flat finish 
is  necessary,  since  no  surfacing  by  flatting  with  pumice,  etc., 
is  permissible  on  account  of  the  comparative  thinness  of  the 
layer.  The  point  as  to  the  necessity  for  application  of 
enamel  to  a  flat,''  i,e.  granular  or  toothed  surface,  should 
be  emphasized  here,  as  it  is  a  well-known  fact  that  com- 
paratively impermeable  coatings,  such  as  glossy  enamel  or 
varnish,  do  not  hold  or    key    well  to  glossy  undercoats. 

The  final,  or  protective,  layer  merits  special  consideration. 
A  system  of  coatings  has  been  described,  so  chosen  that  their 
successive  application  furnishes  practically  a  continuous 
layer  intimately  bound  with  and  extending  into  the  cells  of 
the  wood,  of  such  a  thickness  that  the  surface  imperfections 
of  the  ground  on  which  they  are  applied  is  completely  covered 
and  of  a  solidity  and  hardness  consistent  with  the  require- 
ments for  which  the  article  coated  is  to  be  subjected.  In 
addition,  a  surface  layer,  has  been  obtained  of  a  richness  of 
pigmentation  only  limited  by  the  intrinsic  physical  pro- 
perties of  the  pigments  contained  in  the  colour  coat.  In 
consideration  of  the  weather-resisting  and  elastic  properties 
of  oil  varnishes,  it  might  be  concluded  that  a  layer  of  such 
would  afford  the  degree  of  protection  against  weather  and 


144    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


wear  which  is  required  to  complete  the  structure,  and  such 
it  would  prove  to  be,  were  it  not  for  the  fact  that  in  many- 
colours  or  schemes  of  decoration  the  delicacy  of  the  tints 
are  such  that  the  colour,  conferred  by  the  succeeding  coat  of 
oil  varnish,  would  be  sufficient  to  destroy  the  purity  of  the 
colour  effect  desired.  At  the  same  time,  it  must  be  pointed 
out,  that  in  the  high-class  practice  of  the  coach  painter, 
when  pale  colours  or  delicate  tints  are  not  at  issue,  finishing 
by  means  of  varnishes  is  usually  practised. 

The  use  of  the  finishing  coat  of  glossy  enamel,  however, 
finds  its  main  application  in  the  case  of  the  pale  and  delicate 
tints  referred  to,  white  enamel  being  an  important  variety 
and  forming  a  class  of  its  own.  The  finishing  enamel,  there- 
fore, consists  of  a  medium  possessing  special  properties  of 
weather-resistance  and  flow,  pigmented  to  just  such  a  degree 
that  the  natural  yellow  or  amber  colour  of  the  varnish  is 
overcome.  There  are  many  difficulties  connected  with  the 
manufacture  of  such  a  product  which  are  readily  realized 
when  it  is  considered  that  of  all  the  white  pigments  for  use 
in  white  enamel,  pure  oxide  of  zinc  alone  is  suitable  on 
account  of  its  fineness  of  subdivision  and  susceptibility  to 
forming  to  a  great  extent  a  colloidal  suspension  in  the 
medium.  Zinc  oxide,  being  a  very  actively-basic  pigment, 
readily  forms  combinations  with  the  gum-resins  existing  in 
oil  varnishes,  with  resultant  increase  in  viscosity  in  many 
cases  to  a  degree  rendering  such  pigmented  varnish  unsuit- 
able for  use.  There  are,  in  addition,  certain  physical 
desiderata  of  enamels  connected  with  their  flow,  on  applica- 
tion, etc.,  which  can  hardly  be  discussed  here,  and  which 
make  the  manufacture  of  such  products  a  branch  usually 
left  to  an  expert.  In  general,  however,  it  may  be  stated 
that  the  composition  of  glossy  enamels  lie  in  the  following 
directions : — 

The  Medium. — ^U'hilst  in  ordinary  ready-mixed  paints 
the  media  consist  of  raw  or  boiled  linseed  oil,  such  products 
would  not  possess  the  necessary  physical  or  chemical  pro- 
perties required  in  an  enamel  for  finishing.  The  instability 
of  oxidized  raw  or  boiled  linseed  oil   has  already  been 


PIGMENTS  AND  PAINTS 


145 


discussed,  and  a  consideration  of  the  figures  for  variation 
in  weight  with  time  show  that  disintegration  of  the  dry 
film  sets  in  very  rapidly  after  the  maximum  increase  in 
weight  due  to  oxygen  absorption  has  taken  place.*  In 
addition  to  this,  the  comparatively  low  viscosities,  or,  rather, 
the  surface  tensions  of  both  raw  and  boiled  oil,  would  not 
result  in  that  freedom  of  fiow  on  application  of  the  enamel 
that  is  desired  in  enamel  surfaces.  Thus,  relatively  greater 
stability  to  atmospheric  influences,  together  with  increased 
viscosity  and  surface  tension,  are  attained  by  the  employment 
of  a  medium  in  which  the  oil  present  is  more  or  less  in  the 
form  of  polymerized  molecules.  Such  would  be  found  in 
both  oil  varnishes  and  linseed  oil  thickened  by  heat  stand 
oil'').  On  account  of  the  high  acidity  of  the  gum-resin 
present  in  oil  varnishes,  and  the  basicity  of  the  zinc 
oxide  pigment  which  is  exclusively  used  for  the  pigmenta- 
tion of  the  best  white  enamels,  a  medium  consisting  wholly 
or  almost  entirely  of  stand  oil  is  to  be  preferred,  as  the 
reaction  between  the  acid  resin  and  the  basic  pigment, 
resulting  as  it  does  in  undue  increase  in  viscosity  of  the 
medium,  needs  to  be  regulated  by  further  addition  of  volatile 
solvent  with  consequent  low  volume-concentration  of  fixed 
medium  in  the  film  applied. 

Certain  manufacturers,  however,  employ  gum  varnishes 
containing  kauri  gum  for  their  media,  the  resin  from  such 
having,  in  contradistinction  to  others,  quite  a  low  acid  value 
after  running or  fusion.  Of  recent  times,  processes 
have  been  patented  for  obtaining  neutral  gum-resins  for 
varnish  making  by  forming  glycerin  esters  of  the  fused  gum- 
resins,!  whilst  certain  manufacturers  have  been  successful 
in  employing  glycerin  esters  of  colophony  as  substitutes  for 
gum-resins.  In  general,  however,  stand  oil hardened  with 
a  small  proportion  of  elastic  gum  varnish  is  used,  although 
many  products  are  on  the  market  in  which  the  medium 
consists  of  specially- treated  stand  oil devoid  of  gum-resin. 
The  principal  advantage  of  the  media  of  the  pure  stand  oil  " 
type  is  in  their  need  for  a  lower  proportion  of  volatile  thinner 

*  Cf»  pp.  41  and  45.  -f  Eng.  Pats.  23054  and  23,055  of  1914. 

S.  10 


146    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


to  attain  a  similar  viscosity  when  compared  with  gum- 
resinous  media.  This  latter  factor  in  the  media  of  the  stand 
oil type  reacts  favourably  on  the  flow  of  the  coating  applied, 
in  addition  to  resulting  in  a  thicker  film  being  obtained 
after  evaporation  of  the  volatile  solvent. 

The  Pigment. — In  the  case  of  white  or  pale-coloured 
enamels  derived  from  white,  the  pigment  used  is  exclusively 
zinc  oxide,  the  porcelain-like  effect  of  the  dried  coating 
being  unobtainable  with  any  other  white  pigment  for  the 
reasons  previously  stated.  To  obtain  fair  opacity  with 
overcoming  of  the  slight  colour  of  the  medium  to  obtain 
a  pure  white,  a  degree  of  pigmentation  in  the  neigh- 
bourhood of  65-75  parts  of  medium  (free  from  volatile 
thinners)  to  100  parts  of  zinc  oxide  is  necessary,  a  lower 
degree  of  pigmentation  yielding  a  poor  white  and  a  soft 
film,  whilst  a  higher  degree  impairs  the  gloss  and  flow  of 
the  enamel. 

The  case  of  enamels  of  a  colour  other  than  white  and 
tints  derived  therefrom  would  require  separate  considera- 
tion for  each  pigment,  and  is  beyond  the  scope  of  this  work. 
Certain  general  principles,  however,  hold  over  the  whole 
range  of  pigments.  Thus,  the  maximum  degree  of  pig- 
mentation referred  to  above  must  not  be  exceeded.  The 
activity  of  the  pigments  u§ed  needs  to  be  very  specially 
considered,  particularly  those  which  have  a  tendency  to  accele- 
rate oxidation,  since  an  undue  rate  of  oxidation  in  a  highly 
glossy  film  will  manifest  itself  as  a  loss  of  gloss  due  to  a  micro- 
scopic reticulation  of  the  surface  owing  to  volume  changes, 
etc.,  before  any  appreciable  deterioration  of  elasticity,  etc., 
takes  place.  Another  cause  of  loss  of  permanence  in  gloss 
is  oil-solubility  bleeding ")  of  certain  lake  pigments, 
notably  that  of  madder.  The  relatively  low  opacity  or 
tinctorial  power  of  certain  pigments  is  also  a  source  of 
difficulty,  since  the  degree  of  pigmentation  has  to  be  main- 
tained strictly  within  a  limit. 

The  Volatile  Thinners. — Little  need  be  considered  here 
in  particular  relation  to  enamels.  American  turpentine  is 
the  solvent|usually  chosen  on  account  of  its  better  properties 


PIGMENTS  AND  PAINTS  147 


of  volatility,  small  distillation  range,  and  solvent  action 
over  that  of  its  substitutes.  The  higher  cost  need  hardly 
be  considered  in  view  of  the  comparatively  low  proportion 
present  in  enamels. 

Manufacture  of  Paints  and  Enamei^s 

From  the  foregoing  description  of  the  functions  of 
paints,  it  will  readily  be  conceded  that  to  obtain  those  pro- 
perties of  smoothness  permitting  of  their  employment  in 
the  manner  described,  the  question  of  efficient  subdivision 
and  thorough  amalgamation  of  the  constituent  ingredients 
will  be  paramount.  From  elementary  physical  considera- 
tions, it  will  be  apparent  also  that  the  most  perfect  amalga- 
mation of  solid  and  liquid  phases  would  be  obtainable  by 
minute  subdivision  of  the  pigment,  this  condition  favouring 
the  formation  of  a  maximum  surface  of  the  latter.  Thus, 
the  functions  of  paint-making  machinery  are,  firstly,  that 
of  the  obtainment  of  solid/liquid  pastes  of  as  high  a  degree 
of  dispersion  of  the  former  as  possible,  i.e,  fineness  of 
grinding,  and,  secondly,  even  distribution  of  the  solid  phase 
in  the  liquid,  i.e.  thorough  mixing. 

The  paint  manufacturer  proper  is  not  concerned  with 
the  business  of  the  preparation  of  his  pigment  in  a  suitable 
state  for  grinding  into  paint,  this  being  the  domain  of  the 
pigment  maker,  hence  paint  makers'  pigments  arrive  on  the 
market  in  a  more  or  less  finely  divided  condition.  The 
grinding  of  pigment  in  the  medium,  however,  has  for  its 
object  that  of  the  breaking  down  of  agglomerates  of  fine 
particles  and  the  isolation  of  the  latter  from  each  other  by 
a  skin  or  layer  of  medium.  Such  an  effect  cannot  be  obtained 
even  with  the  finest  of  pigments  by  capillarity  alone,  a  con- 
siderable degree  of  pressure  being  necessary  to  obtain  the 
effect.  Hence,  a  consideration  of  the  principle  underlying 
the  various  types  of  plant  used  for  grinding  pigments 
in  oil  will  show  that  in  every  case  the  effect  aimed  at 
is  the  bringing  together  under  pressure  of  a  thin  layer 
of  pigment  and  medium.  Generally  speaking,  the  manu- 
facture of  the  finished  paint  from  the  raw  materials, 


148    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


pigment,  oil,  and  volatile  thinner,  divides  itself  into  three 
stages  : — 

(1)  The  preliminary  incorporation  of  the  solid  pigment 
with  the  oil  to  form  a  paste  of  pigment- agglomerate  with 
oil.  In  more  modern  machinery,  the  agglomerate  is  partly 
broken  down  in  the  preliminary  stage. 

(2)  The  subdivision  of  the  oil  pigment-agglomerate  to  a 
finer  state.  This  may  be  accomplished  in  one  or  two  stages. 

(3)  The  dilution  or  thinning  of  the  paste  to  a  fluid  paint 
of  workable  consistency.    It  is  obvious  that  it  would  be 


Fig.  16. — Torrance  Sc  Sons'  "  Perfect  "  Mixer. 


inexpedient  to  perform  the  actual  grinding,  as  in  Stage  II., 
to  a  fluidity  other  than  the  stiffest,  if  only  on  account  of 
the  lesser  weight  of  material  to  be  treated. 

The  mechanism  of  the  operations  involved  will  best  be 
understood  by  reference  to  descriptions  of  the  machinery. 

Stage  I. — Preliminary  Mixing  of  the  Pigment  with  Oil. 
In  this  stage  the  mechanism  of  the  process  consists  in  the 
forcing  of  the  cohesion  and  capillary  attraction  between  the 
medium  and  the  pigment  agglomerates  without,  however, 


PIGMENTS  AND  PAINTS  149 


any  appreciable  subdivision  of  the  latter.  In  one  type  of 
machine  for  this  purpose  (Fig.  17),  however,  some  disintegra- 
tion of  agglomerates  is  inaugurated.  In  the  ordinary  old- 
type  pug-mill  the  dry  pigment  is  charged  into  the  pan,  the 
radial  arms  disposed  about  the  vertical  shaft  having  their 
''set''  alternately  oppositely  inclined,  thus  effecting  local  com- 
pression of  oil  and  pigment  particles.  A  mixer  embodying 
an  improvement  of  the  pug-mill  principle  is  shown  in  Fig.  16. 
In  the  pan-mill  or  positively-driven  edge-runner  (Fig.  17),  the 


Fig.  17. — Torrance  &  Sons' Positive-Driven  Edge- Runner  or  "  Pan  "  Mill. 

pressing  or  squeezing  action  is  self-explanatory,  and  the 
differential  speeding  occurring  on  that  part  of  the  periphery 
of  the  runner  in  contact  with  the  material  in  the  bed  of  the  pan 
is  subjected  to  a  slip  increasing  in  intensity  from  a  zero  at  the 
middle  point  of  the  periphery  face  to  a  maximum  at  its  edge, 
whereby  a  considerable  degree  of  subdivision  takes  place. 

Stage  II, — Grinding.  The  triple  granite  roller  mill  has, 
in  modern  times,  supplanted  every  other  type  of  grinding 
mill  for  reduction  of  a  pigment-oil  mixture  to  a  smooth 


150    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


butter-like  paste,  both  on  account  of  its  rapidity  and  the 
perfection  of  the  results  obtained.  In  the  Torrance  Patent 
.Silent  Quadrant  Roller  Mill  (Fig.  i8)  we  have  a  combination 
of  three  granite  rollers  geared  at  differential  and  increasing 
speeds  from  back  to  front  (feed  to  delivery)  through  quad- 
rants, i.e,  intermediate  spur  wheels,  enclosed  in  oil-baths, 
thus  allowing  for  wear  of  grinding  surfaces  without  corre- 
sponding bedding  of  their  respective  gears.  A  lateral 
motion  is  imparted  to  the  middle  roller  by  a  differential 


Fig.  i8. — Torrance     Patent  Silent  Quadrant  "  Roller  Mill. 


gear  similar  to  that  on  the  rear  axle  of  an  automobile,  thus 
ensuring  even  wear  of  the  rolls  and  avoidance  of  ridges. 
The  material  from  the  mixer  (Stage  I.)  is  fed  by  hand  or 
from  a  hopper  on  to  the  back  (slowest)  roller,  whence  it 
travels  by  contact  to  the  middle,  and  finally  to  the  front 
(fastest)  roller,  a  ductor  or  scraper  removing  it  after  treat- 
ment. Fig.  19  shows  the  Torrance  Combination  Mill 
embodying  the  pan-mill  automatically  delivering  to  a  pair 
of  roller  mills  in  tandem.    It  is  necessary  to  remark  that  a 


PIGMENTS  AND  PAINTS 


combination  of  more  than  three  rollers  in  a  roller  mill 
has,  in  general,  not  been  found  expedient  owing  principally 
to  the  resulting  overcoming  of  adhesion  by  centrifugal 
force  on  the  delivery  roll  in  consequence  of  its  high  speed. 
It  is  to  be  noted  that  the  roller  mill  described  is 
designed  for  treating  a  paste  as  distinct  from  a  paint  of 
workable  consistency.  This  is  no  disadvantage,  since 
reduction  of  a  ground  paste  to  a  working  consistency  is 
easily  accomplished  by  simple  agitation  without  further 


Fig.  19. — Torrance  &  Sons'  Patent  "  Combination  "  Mill. 

disintegration,  and,  moreover,  such  treatment  in  the  paste 
form  is  actually  advantageous  from  the  point  of  view  of 
rapidity  of  output. 

For  the  satisfactory  grinding  to  a  stiff  paste  of  pigments 
in  two  mutually  immiscible  media,  i.e.  oil  and  water,  such 
as  obtains  in  patent  driers,''  the  fiat  stone  mill  consisting 
of  two  granite  discs  superposed  in  a  horizontal  plane,  the 
lower  one  being  fixed  and  the  upper  being  revolved,  is  used. 
The  contact  faces  of  the  stones  are  grooved  in  order  to  allow 


152    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 

passage  of  the  material  from  the  centre  feed  to  the  delivery 
at  the  circumference. 

Although  the  triple  granite  roller  mill  has  practically- 
supplanted  every  other  type  of  mill  for  the  purpose,  the 
cone  mill  (Fig.  20)  still  finds  some  favour,  mostly  for  the 
preparation  of  small  trial  batches  of  paints  and  enamels, 
although  some  manufacturers  still  retain  it  under  an  impres- 
sion as  to  its  greater  efficacy  in  producing  subdivision,  In 
general,  however,  it  may  be  stated  that,  apart  from  laboratory 

use,  its  main  function 
lies  in  the  direction  of 
the  grinding  of  pigments 
in  volatile  media,  e.g. 
spirit  varnish  paints, 
pigmented dopes,''  etc., 
where  the  large  surface 
exposed  on  the  roller  mill 
would  be  inadmissible. 
The  new  Disc  ''  mill  of 
Messrs.  Torrance  &  Sons 
(Fig.  21)  is  an  improve- 
ment on  the  cone  mill 
principle,  in  that  both 
a  rotary  and  an  eccentric 
motion  is  imparted  to 
one  of  the  grinding  sur- 
faces of  the  granite 
stones. 

Stage  III.  Apart 
from  the  consideration  of  economy  and  speed  of  output, 
the  dilution  "  of  the  ground  paste  paint  to  the  finished 
product  can  well  enough  be  accomplished  by  hand.  One 
type  of  mixer  (see  Fig.  16)  which  has  been  previously  re- 
ferred to  as  a  mixer  for  amalgamating  dry  pigment  and 
medium,  is  equally  applicable  for  thinning  the  paste  to  a 
working  consistency. 

The  degree  of  treatment  necessary  to  produce  pastes  of 
a  sufficient  degree  of  subdivision  depends  on  both  the 


PIGMENTS  AND  PAINTS 


153 


nature  of  the  pigment  and  the  purpose  of  the  product.  An 
insufficiency  of  grinding,  resulting  in  the  appearance  of 
aggregate  particles  of  larger  size  than  in  the  bulk,  will 
obviously  not  have  such  an  importance  in  a  paint  intended 
for  the  protection  of  farm  buildings,  etc.,  as  in  that  for  a 
glossy  enamel.  Nevertheless,  uniform  and  fine  subdivision, 
resulting  as- it  does  in  the  presentation  of  a  minimum  of 


Fig.  21. — -Torrance  &  Sons'  Patent  "  Disc  "  Mill. 


surface  for  a  given  area  covered,  always  tends  towards  more 
adequate  protection. 

The  manufacture  of  enamels  does  not  differ  in  kind 
from  that  of  paints  excepting  that  the  grinding  treatment 
in  the  former  case  is  always  carried  to  a  greater  degree,  and 
correspondingly  more  care  is  exercised  in  the  manufacture. 
Since,  however,  certain  types  of  enamels  require  grinding 
in  a  special  medium,  often  containing  an  appreciable  quantity 
of  volatile  thinner,  the  plant  used  for  grinding  must  be 


154   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


chosen  with  due  regard  to  the  minimization  of  loss  of 
thinner  by  evaporation.  In  extreme  cases,  the  cone  mill, 
with  its  lesser  area  of  grinding  surfaces  exposed  during 
operation,  must  be  used  in  place  of  the  roller  mill. 

Since,  as  stated  above,  paints  are  products  intended 
to  perform  all  the  functions  desired  in  a  coating,  as  dis- 
tinguished from  enamels  in  which  each  function,  i.e.  filling, 
priming,  undercoating,  and  finishing,  is  represented  separately 
over  a  number  of  coats,  the  composition  of  the  former 
products  requires  somewhat  special  consideration.  Thus  a 
paint  should  possess  reasonable  opacity,  tinctorial  power, 
flow,  viscosity,  and  surface  tension,  and  the  attainment  of 
all  these  objects  in  one  product  is  often  a  matter  of  some 
difficulty.  The  properties  of  the  common  pigments  have 
already  been  referred  to  (see  chapter  on  Pigments) ,  so  that 
a  recapitulation  of  the  principles  governing  the  choice  of 
ingredients  in  a  paint  will  be  all  that  need  be  recalled  at 
this  juncture.  Without  purporting  to  form  any  but  a  very 
general  classification,  the  subdivision  of  paints  under  the 
headings  of  their  various  basic  colours  will  form  a  scheme  for 
their  consideration  with  regard  to  the  principl  es  referred  to. 

White  Paints, — Since  in  pigments  of  this  colour,  opacity 
and  staining  power,  although  not  synonymous  or  necessarily 
of  the  same  order,  are  always  present  together,  a  white 
paint  may  consist  of  one  or  a  mixture  of  the  common  white 
pigments,  raw  or  boiled  linseed  oil,  drier  and  volatile 
thinner.  The  addition  of  inert pigments,  other  than  in 
a  proportion  necessary  to  confer  tooth or  crystalline 
structure  to  an  otherwise  too  soft  pigment,  e.g.  zinc  oxide, 
must  be  looked  upon  as  an  adulteration.  As  detailed  under 
Manufacture  of  Paint,''  the  paint  in  its  paste  form  is  thinned 
to  a  working  consistency  ready-mixed  paint  ")  by  means 
of  more  oil  and  volatile  thinners.  These  latter  additions 
must  be  made  with  due  regard  to  the  physical  properties 
desired  in  the  finished  product,  too  small  an  addition  of  oil 
resulting  in  a  deficiency  of  flow,  whilst  too  much  oil  added 
results  in  poorness  of  body,  etc.  Generally  speaking,  the 
amount  of  thinner  in  the  finished  paint  is  a  fixed  proportion 


PIGMENTS  AND  PAINTS  i55 


relative  to  the  total  oil  present.  The  addition  of  thinner 
regulates  the  thickness  of  the  film  applied  by  varying  its 
viscosity.  The  proportion  of  drier  to  be  added  to  a  paint 
will  be  dependent  on  the  drying  power  desired  and  the 
intrinsic  enhancing  or  inhibiting  properties  of  the  pigment. 

Tinted  "  white  pigments,  i.e.  pale  tints,  are  usually 
prepared  by  addition  of  the  tinting  pigments  ground  in 
paste  form  to  the  ready-mixed  paint. 

Red  and  Brown  Paints.— The  bright  red  paints,  e.g. 
''Post  Office  Red,"  ''Signal  Red,"  "Cardinal  Red,"  etc., 
are  obtained  by  dependence  for  tinctorial  power  on  the 
lake  pigments  (q.v.),  vermilion  finding  application  only  in 
special  cases,  owing  to  certain  undesirable  properties  which 
it  possesses  and  to  its  expense.  The  red  lake  pigments,  how- 
ever, although  of  high  staining  powder,  possess,  as  a  rule, 
relatively  little  opacity,  since  the  majority  of  them  are 
struck  on  a  transparent  base,  i.e.  blanc  fixe,  alumina,  etc. 
The  opacity  of  these  pigments,  however,  may  be  increased 
by  precipitation  of  the  dye  on  a  base  of  high  opacity  such 
as  orange  lead,  white  lead,  or  lead  sulphate.  The  employ- 
ment of  lakes  precipitated  in  the  latter  manner  is  the 
expedient  usually  resorted  to  in  the  manufacture  of  paints, 
but  the  presence  of  lead  in  the  base  mitigates  against  their 
successful  employment  in  glossy  enamels.  Increased  opacity, 
therefore,  in  the  latter  class  of  product  is  usually  obtained 
by  employment  of  a  red  lake  precipitated  on  a  comparatively 
transparent  base  in  conjunction  with  a  proportion  of  inert 
filler  such  as  blanc  fixe,  having  for  its  object  the  pro- 
duction of  a  thicker  coating  of  increased  opacity  without  the 
corresponding  alteration  of  tone  which  would  be  obtained 
by  addition  of  an  opaque  pigment  such  as  white  lead,  etc. 

The  question  of  "  bleeding  "  of  certain  red  lake  pigments 
applies  rather  more  in  the  case  of  glossy  enamels,  as  the 
solubility  of  the  organic  dye  often  adversely  influences  the 
gloss,  rate  of  drying,  etc. 

Whilst  the  production  of  satisfactory  paint  and  enamel 
products  of  bright  red  colour  is  one  of  the  most  difficult 
propositions  in  the  paint  industry  owing  to  the  somewhat 


156    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


unsatisfactory  nature  of  the  dyes  at  our  disposal,  the 
natural  and  artificial  earth  oxides  offer  a  wide  range  of 
products  for  the  production  of  paints  and  enamels  of  a  high 
degree  of  fastness  to  light  and  good  opacity.  The  staining, 
power  and  opacity  of  such  a  pigment  as  Turkey  Red  is  of 
such  a  high  degree  that  considerable  reduction  of  the  same 
with  an  inert  filler  is  expedient.  Intermediate  shades  of 
maroon,  purple  brown,  etc.,  are  obtained  by  combination 
of  lake  reds  with  iron  oxide  pigments. 

The  naturally-occurring  and  artificially-prepared  oxides 
of  iron,  having  contents  of  Fe203  from  lo  per  cent,  upwards, 
form  the  basis  of  the  iron  oxide  paints  that  are  so  largely 
used  in  the  protection  of  iron  structures,  their  permanence 
and  supposed  rust-inhibiting  effects  making  them  especially 
popular  in  this  respect.  It  should  be  stated,  however,  that 
some  varieties  of  iron  oxide  reds  prepared  by  ignition  of  iron 
sulphate  are  liable  to  retard  drying  of  paints,  probably 
owing  to  their  persistent  retention  of  free  sulphuric  acid 
and  soluble  salts.  The  addition  of  basic  lead  chromate 
(''American  vermilion''),  zinc  chromate,  red  lead,  and 
zinc  oxide  to  iron  oxides  is  stated  to  render  these  paints 
rust  inhibitive.* 

Yellow  Paints. — The  bright  yellow  paints  are  almost 
exclusively  prepared  from  lead  chromates.  The  chromates 
of  zinc  and  barium  are  never  used  alone  for  the  production 
of  yellow  paints  on  account  of  their  relatively  low  opacity 
as  compared  with  those  of  lead.  Owing  to  the  unnecessary 
thickness  of  coating  applied  and  their  high  opacity,  lead 
chromates  are  commonly  reduced  with  inert  pigments  such  as 
barytes,  in  the  manufacture  of  paints.  They  are  usually  good 
driers  and  possess  good  permanency,  excepting  in  the  presence 
of  acid  or  sulphurous  fumes,  a  failing  common  to  all  the  lead 
pigments. 

The  naturally-occurring  ochres  place  at  the  disposal  of 
the  paint  manufacturer  a  wide  range  of  pigments  for  the 
production  of  paints  and  enamels  of  a  more  subdued  yellow 
than  those  described  above.    Some  varieties  mined  in  Italy 

*  Cf.  Gardner,  "  Paint  Researches,"  T917,  p.  116. 


PIGMENTS  AND  PAINTS 


157 


possess  exceptionally  high  staining  powers  and  opacities, 
the  tones  obtained  by  their  reduction  with  pigments  of  high 
opacity  closely  simulating  those  obtained  from  the  chrome 
yellows.  The  yellow  ochres  represent  perhaps  the  most 
permanent  of  pigments. 

Green  Paints. — ^The  various  shades  of  green  in  paints 
are  almost  exclusively  obtained  by  a  combination  of  blue 
and  yellow  pigments,  the  pigment  compounds  possessing 
a  green  colour,  per  se,  viz.  emerald  green  and  oxide  of 
chromium  finding  very  restricted  application  for  reasons 
already  explained.  The  brighter  shades  of  green  are 
obtained  by  combination  of  lead  chromates  and  Prussian 
blues,  the  high  opacity  of  the  former  and  high  staining 
power  of  the  latter  particularly  fitting  the  combination  as  a 
strong  compound  pigment.  Greens  made  on  such  basis 
C  Brunswick  greens  are  fairly  fast  to  light,  the  fugitive 
component  being  the  Prussian  blue,  which  has  a  tendency 
to  bleach  on  exposure  to  light,  the  original  colour,  however, 
returning  in  the  dark.  The  duller  shades  of  green,  viz. 
olive  green  and  bronze  green  are  obtained  by  substitution 
of  chromates  of  zinc  or  barium  in  the  one  case,  and  bright 
yellow  ochres  in  the  other,  in  place  of  lead  chromate. 

Blue  Paints. — ^The  colour  of  these  paints  is  exclusively 
obtained  from  ultramarine  and  Prussian  blues.  Neither 
pigment  possesses  a  high  opacity,  and  since  alteration  in  tone 
to  a  muddy  colour  results  by  reduction  with  opaque  white 
pigments,  it  is  the  practice  to  obtain  satisfactory  coatings 
by  reduction  with  inert  transparent  fillers,  the  result  of 
which  is  to  increase  the  thickness  of  film  applied.  The 
intensity  of  colour  (depth  or  low  light-reflecting-power)  of 
these  pigments  being  very  great,  reduction  is  always  prac- 
tised, as  otherwise  the  quality  of  the  colour  is  not  apparent. 
The  varieties  of  ultramarine  and  ferrocyanogen  blues  place 
at  the  disposal  of  the  paint  manufacturer  a  wide  range  of 
tones  ranging,  in  the  case  of  ultramarine,  from  a  greyish-blue 
to  a  reddish-purple,  and  in  that  of  the  ferrocyanogen  blues 
from  a  greenish-blue  to  a  deep  blue  of  pronounced  bronze 
tint.    Paints  containing  much  ultramarine  blue  are^often 


158  RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


found  to  be  bad  driers/'  the  precise  reason  for  which  is 
not  accurately  known,  the  suggestion  of  the  presence  of 
free  sulphur  being  a  popular  explanation.  Ferrocyanogen 
blues,  however,  are  good  driers,  the  slight  solubility  of  iron 
in  the  medium  used  being  a  probable  explanation. 

Black  Paints. — ^The  several  black  pigments  used  in 
paint  and  enamel  manufacture  have  already  been  described. 
The  black  pigments  all  possess  the  general  characteristic 
of  comparatively  high  staining  powers  when  compared  with 
other  staining  pigments,  in  addition  to  which  they  possess 
considerable  opacity.  These  characteristics,  taken  in  con- 
junction with  their  very  high  oil  absorptions,  make  it 
inexpedient  to  employ  them  as  sole  pigments  in  paints. 
They  are  therefore  used  in  conjunction  with  other  pigments. 
Indeed,  their  high  staining  power,  generally  speaking,  is 
such  that  it  is  possible  to  incorporate  quite  large  proportions 
of  opaque  pigments  such  as  white  lead  into  black  paints. 
The  high  degree  of  perfection  of  flow  required  in  glossy 
enamels  makes  it  necessary  to  exercise  great  care  in  the 
selection  of  black  pigments  for  these  preparations,  the  large 
proportion  of  adsorbed  oily  impurities  in  lamp  black  having 
a  very  adverse  effect  on  the  flow  and  drying  properties  of  an 
enamel  containing  it. 

Commercial  Paints. — ^When  speaking  of  paints  in  the 
foregoing  pages,  ready-mixed  paint  or  paint  of  a  con- 
sistency ready  for  application  containing  the  necessary 
driers  is  understood.  Nevertheless,  the  larger  users,  such 
as  the  public  bodies,  railway  companies,  etc.,  are  in  the 
habit  of  purchasing  paint  in  the  state  of  stiff  paste,  the 
ultimate  users,  i.e.  the  craftsmen  employing  the  material, 
thinning  down  the  paste  themselves  to  a  working  consistency 
with  oil  and  thinners  and  adding  the  necessary  driers.  It 
has  already  been  shown  that  the  necessary  grinding  takes 
place  entirely  in  the  stage  preliminary  to  thinning,  so  that 
such  a  practice  is  of  no  disadvantage  to  the  consumer,  and 
shows  advantages  in  both  economy  of  transport  and  possi- 
bility of  storage  without  loss  by  evaporation  or  formation 
of  skins,  as  driers  are  usually  not  present  in  the  paste  form 


PIGMENTS  AND  PAINTS  159 


of  paint.  Such  of  the  coach  painting  trade  as  have  not 
adopted  the  system  of  decorating  by  enamels,  also  purchase 
paint  in  the  paste  form,  whereby  they  are  enabled  to  thin 
down  to  a  working  consistency  with  any  medium  they  choose. 

In  regard  to  flat-drying  paints,  the  ordinary  paste  paint 
referred  to  would  be  unsuitable,  as  the  proportion  of  oil 
carried  would  be  in  excess  of  that  which  would  dry  with  a 
matt  surface  even  if  the  necessary  thinner  were  to  consist 
exclusively  of  volatile  solvent.  For  such  purpose,  paste 
paints  are  produced  in  which  the  grinding  medium  consists 
of  a  mixture  of  quick-drying  varnish  (goldsize)  and  volatile 
thinner.  Such  paints  are  variously  known  as  turps 
colours,''  ready-bound  colours,''  or  colours  ground  in 
turpentine."  On  account  of  their  deficiency  in  non- volatile 
binding  medium,  care  has  to  be  exercised  that  undue 
evaporation  does  not  occur,  or  difiiculty  would  be  experienced 
in  subsequently  diluting  to  a  thin  consistency. 

Distempers  and  Water  Paints 

When  decoration,  or  more  strictly  speaking,  coloration 
of  a  surface  without  corresponding  protection  to  an 
appreciable  degree  is  desired,  the  employment  of  a  dis- 
temper or  water  paint  is  resorted  to.  Such  cases  would 
arise  in,  e.g.  interior  decoration  of  walls,  panels,  etc.  It 
is  obvious  that  when  mere  coloration  or  obscuration 
of  a  surface  is  desired,  the  question  as  to  the  relation 
between  pigmentation  and  impermeability  will  not  arise 
in  so  far  as  the  application  of  the  principle  applies  to  the 
securing  of  impermeability.  Thus,  pigmentation  can  be 
carried  to  a  degree  only  limited  by  the  binding  power  of 
the  medium,  i.e.  the  proportion  of  medium  to  pigment  in 
the  dried  film  need  only  be  the  minimum  necessary  to  secure 
cohesion.  Bearing  this  fact  in  mind,  it  is  evident  that  the 
problem  of  the  composition  of  a  decorative  paint  is  some- 
what simpler  than  that  of  one  in  which  impermeability, 
etc.,  is  necessary,  by  reason  of  the  important  fact  that  the 
medium  in  the  dried  coating  being  at  a  minimum,  the  influ- 
ence of  closeness  of  refractive  index  of  medium  to  that  of 


i6o    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


pigment  (see  above,  p.  109)  in  producing  lack  of  opacity 
will  not  be  so  great.  To  make  this  clear,  we  may  consider 
the  extreme  case  of  a  film  of  pigment  quite  devoid  of  medium. 
Granting  optical  discontinuity  in  the  particles  of  pigments 
themselves  by  great  subdivision,  we  have  therefore  to  con- 
sider an  optical  system  air/pigment,  the  low  refractive  index 
of  the  former  in  comparison  with  that  of  the  pigment  which 
we  may  take  as  being  no  lower  than  1*4,*  acting  for  the 
attainment  of  the  maximum  opacity  or  optical  discontinuity. 
The  increase  of  opacity  taking  place  as  a  film  of  whiting 
and  water  dries  is  thus  explained.  It  should  be  remarked, 
however,  that  in  consequence  of  the  fact  that  such  paints 
contain  a  proportion  of  non- volatile  medium  below  that  of 
their  specific  absorption,  the  dried  films  obtained  therefrom 
will  be  devoid  of  gloss. 

The  degree  of  binding  medium,  i.e.  the  reverse  of  rela- 
tive pigmentation,''  varies  considerably  in  the  several 
varieties  of  water  paints  used  commercially.  lyimewash  " 
and  whitewash  ''  may  be  taken  as  being  representative  of 
the  lowest  members  of  the  class,  being  merely  aqueous 
suspensions  of  lime  and  whiting  respectively.  Lime- 
wash  "  is  used  where  disinfectant  effect  together  with 
obscuration  is  aimed  at,  e.g,  on  the  walls  of  cow-sheds,  etc. 

Whitewash  "  is  used  where  temporary  obscuration  of  a 
surface  is  desired,  e.g.  glass,  ceilings,  etc.  However,  neither 
of  these  products  can  strictly  bear  the  appellation  of  water 
paints,  owing  to  the  absence  of  binding  media  in  the  dried  film. 

The  introduction  of  a  proportion  of  binder  together 
with  an  attainment  of  those  properties  of  increase  of 
viscosity,  interfacial  (pigment /medium)  tension,  etc.,  which 
characterize  a  suitable  product  are  in  one  class  of  water 
paint  obtained  by  the  use  of  glue,  glue-size,  gum,  dextrine, 
or  starch.  Such  a  paint  results  in  a  more  evenly-distributed 
coating  of  greater  thickness  than  if  water  were  used  alone, 
and  under  most  conditions  obtaining  in  protected  situations, 
of  great  permanency.  Comparatively  little  binder  is  neces- 
sary for  a  large  amount  of  pigment,  so  that  the  latter  may 

*  Cf.  Gardner,  **  Paint  Researches/*  191 7,  p.  43. 


PIGMENTS  AND  PAINTS  i6i 

be  chosen  from  a  class  possessing  relatively  low  refractive 
indices,  i.e.  whiting,  China  clay,  etc.,  due  regard  being  had 
to  their  colour.  China  clay  (see  chapter  on  Pigments)  is 
largely  used  on  account  of  its  propert}-  of  remaining  sus- 
pended in  a  water  medium  over  long  periods.  It  is  obvious, 
however,  that  since  no  change  occurs  on  drying  of  the  paint 
applied,  the  dried  film  will  remain  soluble  in  water,  thus 
mitigating  against  its  permanency  in  damp  situations,  or 
permitting  of  its  cleansing  by  water  when  soiled. 

Washable  water  paint,''  or  true  distemper,  belongs  to 
a  class  of  improved  product  on  that  described  above.  The 
name  is  self-explanatory  in  that  these  products  afford 
surfaces  which  are  sufficiently  bound  and  water-insoluble 
to  permit  of  subjection  to  a  mild  washing  treatment.  The 
insolubility  of  the  dried  film  is  commonly  obtained  by 
dependence  on  one  of  two  principles.  In  the  first,  the 
binder  consists  of  a  solution  of  calcium  caseinate,  or  a 
mixture  of  casein  and  slaked  lime.  Such  preparations 
become  insoluble  on  drying  by  reason  of  the  transformation 
of  the  lime  to  carbonate,  thus  leaving  the  casein  in  its 
original  (?)  insoluble  form.  This  latter  class  of  product  owes 
its  popularity  to  the  fact  that  it  is  capable  of  sale  in  a  powder 
form,  addition  of  water  to  the  consistency  desired  being  all 
that  is  necessary  to  render  it  ready  for  use. 

The  principle  of  the  formation  of  an  insoluble  condition 
of  the  dried  film  in  the  second  class  is  somewhat  ingenious, 
although  it  is  probable  that  the  inception  of  the  product 
arose  empirically  without  any  regard  to  the  main  cause  of 
the  function  involved.    These,  the  most  insoluble  form  of 
water  paint,  consist  of  suspensions  of  pigments  in  an  aqueous 
emulsion  of  glue  or  alkaline  casein  solutions  with  drying 
oils.     The  oils  are  usually  present  in  such  proportions 
as  to  be  entirely  kept  in  permanent  suspension  and  not 
visibly  present  in  the  dried  coating.    On  exposure,  the 
film  first  dries  by  evaporation  of  its  water,  a  secondary 
reaction  of  oxidation  of  the  drying  oil  with  evolution  of 
volatile  products  causing  insolubility  of  the  protein  material 
(glue  or  casein)  to  take  place,  and  usually  completing  itself 
S.  II 


i62    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


in  3-4  weeks.  It  is  probable  that  an  aldehyde,  e.g,  formalde- 
hyde, is  the  agent  involved  which  canses  such  insolubility. 
The  dried  coating  becomes  then  insoluble  by  reason  of  both 
the  protecting  influence  of  the  water-insoluble  solid  drying 
oil  and  the  insoluble  form  of  protein.  The  presence  of 
drying  oil  with  a  refractive  index  approaching  that  of 
whiting  and  China  clay,  however,  necessitates  their  complete 
or  partial  substitution  by  those  pigments  which  are  com- 
monly used  in  oil  paints  (zinc  oxide,  lithopone,  etc.) . 

Other  forms  of  washable  water  paints  containing  emulsi- 
fying agents  are  on  the  market,  e.g.  sulphonated  oils,  etc., 
but  their  description  lies  outside  the  scope  of  this  work. 

lyittle  need  be  said  as  to  the  manufacture  of  water 
paints.  Since  the  purpose  for  which  they  are  intended  is 
usually  that  of  the  covering  of  a  surface  not  specially  pre- 
pared (plaster,  brick,  etc.),  the  importance  of  fineness  of 
grinding  is  not  great,  apart  from  the  fact  that  defects  in 
surface  are  far  less  apparent  in  flat-drying  than  glossy 
paints.  Dry  distempers,''  i.e.  those  sold  in  powder  form, 
are  prepared  by  mechanically  mixing  and  sifting  simultane- 
ously through  a  rotating  horizontal  sieve.  Other  distempers 
are  usually  sold  in  paste  form  and  require  thinning  with 
water  only  to  bring  them  to  the  required  consistency.  The 
pigments  are  then  usually  incorporated  with  the  media  by 
grinding  under  an  edge  runner  (see  Fig.  17). 

It  is  to  be  noted  that  on  account  of  the  liability  to 
decomposition  which  aqueous  preparations  of  glue,  casein, 
etc.,  are  subject,  an  antiseptic  such  as  phenol,  borax,  etc., 
is  added  to  the  paste  distemper. 


Section  II.— LINOLEUM,  FLOORCLOTH,  AND 
CORK  CARPET 

IviNOi^EUM,  together  with  its  sister  product,  floorcloth, 
differs  from  paints  and  varnishes  in  its  function  of 
serving  primarily  as  a  mechanical  insulator  and  decora- 
tive coating  and  not,  strictly  speaking,  as  a  protective 
agent  against  decay  and  corrosion.  Its  employment  has 
of  recent  years  assumed  gigantic  proportions,  and  on  account 
of  its  hygienic  properties,  it  is  largely  superseding  carpets, 
especially  for  public  buildings,  shops,  etc. 

lyinoleum  was  first  manufactured  a  little  over  half  a 
century  ago  by  Frederick  Walton  at  Staines,  and  it  is  curious 
to  note  that  in  spite  of  the  manufacture  of  linoleum  being 
now  carried  out  in  different  factories  all  over  the  world, 
little,  if  any,  change  in  the  process  of  manufacture  has  been 
made  since  its  first  inception. 

The  material  is  too  familiar  for  a  description  to  be  given 
in  these  pages,  but  it  may  be  as  well  to  detail  the  various 
forms  under  which  linoleum  and  its  congeners  are  manu- 
factured. 

Linoleum  consists  of  a  composition  of  oxidized  linseed 
oil,  resins,  pigment,  wood  or  cork  fibre,  mounted  on  a  canvas 
backing  usually  painted  on  the  under  side.  It  appears 
in  three  forms:  plain,''  i.e,  unicoloured  ;  inlaid,"  in 
which  a  many-coloured  *'tile,''  floral,  or  carpet  pattern 
extends  right  through  from  the  upper  surface  to  the  canvas  ; 
and  printed,''  in  which  a  superimposed  painted  design  has 
been  impressed  on  a  plain  linoleum.  All  the  varieties 
agree  in  that  the  body  of  composition  is  of  very  definite 

163 


i64    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


thickness,  and  can  be  detached  as  a  whole  from  the 
canvas. 

Floorcloth  is  merely  canvas,  such  as  is  used  for  backing 
linoleum,  with  a  varying  number  of  coats  of  coarse  paint 
applied  to  both  sides,  the  number  of  applications  of  paint 
usually  (in  the  heavier  grades)  predominating  on  the  face  or 
wearing  side.  The  final  face  coat  invariably  bears  a  printed 
design. 

Cork  Carpet  is  somewhat  similar  to  linoleum  in  appear- 
ance, differing,  however,  in  being  far  more  spongy  to  the 
tread  and  usually  rough  in  surface.  It  is  rarely  printed  on, 
or  when  such  is  the  case,  a  light  design  is  applied  leaving  the 
bulk  of  the  groundwork  visible.  A  further  difference  in 
cork  carpet  is  the  fact  that  because  wood  fibre  has  an  inferior 
resiliency  to  cork  dust  the  latter  is  always  used,  and  the 
employment  of  coarse  cork  ensures  such  resiliency  to  a  great 
degree. 

lylNOIvKUM 

As  stated  above,  this  product  consists  of  a  composition 
of  oxidized  linseed  oil,  resins,  cork  or  wood  fibre,  and  pig- 
ment. Linoleum  owes  its  various  properties  to  its  several 
constituents  as  follows  : — 

The  binding  medium  consists  of  oxidized  linseed  oil, 
rosin,  and  kauri  gum.  The  way  in  which  each  of  these 
function  and  their  method  of  preparation  will  be  described 
later. 

The  resiliency,  and  the  sound-  and  heat-insulating 
properties  are  due  to  the  cork  dust  and  wood  flour,  the 
former  being  considerably  the  better  for  the  purpose,  but  on 
accotmt  of  its  dark  colour  it  is  unsuitable  for  the  lighter 
shades  of  inlaid  linoleum  in  any  but  small  proportions  with- 
out the  employment  of  such  a  large  proportion  of  colour- 
killing  pigment  (white)  as  would  diminish  its  resihency 
to  too  great  a  degree. 

The  colour  is  yielded  by  the  pigment.  Cork,  when  com- 
pounded with  the  binding  medium,  gives  a  brown  colour. 


LINOLEUM,  FLOORCLOTH,  CORK  CARPET  165 


The  natural-coloured/'  or  brown,  linoleums  are  the  most 
lightly  pigmented  and  consequently  possess  the  greatest 
resiliency.  As  resiliency  goes  hand  in  hand  with  specific 
gravity,  those  pigments  of  large  volume  or  low  specific 
gravity  are  chosen  whichever  process  is  employed,  the 
mineral  filler  or  cheapener  selected  being  common  whiting 
to  the  exclusion  of  barytes. 

Before  describing  in  detail  the  preparation  and  employ- 
ment of  the  several  constituents  entering  into  the  composi- 
tion of  linoleum,  it  will  be  as  well  to  give  a  brief  note  of  the 
sequence  of  manufacture.  On  looking  at  a  piece  of  linoleum, 
it  is  evident  that  the  several  ingredients  have  been  incor- 
porated during  the  course  of  manufacture  in  a  state  of  greater 
plasticity  than  they  appear  in  the  finished  article.  In 
practice  the  binding  medium  is  employed  in  such  a  state 
of  plasticity  that  whilst  sufficiently  solid  to  maintain 
temporarily  the  form  given  to  the  intermediate  or  green 
stage  of  the  product,  its  adhesion  is  sufficiently  great  to 
ensure  the  matured  "  or  finished  product  being  coherent. 
Relatively  little  increase  in  solidity  from  the  unmatured  to 
the  finished  product  is  required  in  linoleum,  whilst  in  floor- 
cloth, which  is  practically  a  paint,  the  article  is  manufactured 
in  the  state  of  a  liquid  suspension,  becoming  transformed 
when  mature  to  a  hard  solid  similar  to  a  paint  film  of  little 
resiliency. 

The  Binding  Medium. — Broadly  speaking,  two  methods 
are  in  use  for  obtaining  what  is  the  main  basis  of  the  binding 
medium,  viz.  solid  oxidized  linseed  oil.  The  first  and  oldest 
scrim  process)  consists  in  subjecting  a  film  of  siccatized 
linseed  oil  on  a  cotton  fabric  to  atmospheric  oxidation,  the 
solid  oxidized  surfaces  being  successively  employed  as 
undercoats  for  succeeding  coats  until  a  thickness  of  an  inch 
or  more  is  obtained.  This  oxidation  is  carried  out  in  oxi- 
dizing sheds/'  large  buildings  capable  of  accommodating 
2000  or  3000  cotton  fabrics,  20  ft.  by  3  ft.  in  area, 
suspended  vertically  and  flooded  with  oil  by  means 
of  an  overhead  travelling  trough.  The  surplus  oil  drains 
into  gutters  at  the  base  of  the  building  and  circulates 


i66    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


for  further  flooding  to  the  overhead  trough  by  means 
of  a  rotary  pump  (Fig.  22).  A  thorough  flooding  is  given 
every  twelve  or  twenty-four  hours  according  to  the  drying 
activity  of  the  oil  and  is  continued  until  sufflcient  thickness 
of  film  is  considered  to  have  been  obtained.  The  tempera- 
ture in  the  oxidizing  shed  is  maintained  a  little  above  the 
atmospheric  in  order  to  accelerate  drying.  The  further 
treatment  of  the  solidified  oil  is  the  same  in  this  and  the 
newer    shower-bath  "  process  to  be  described. 

The  more  effective  shower-bath  "  process  for  effecting 
solidification  proceeds,  on  account  of  mechanical  expediency. 


Fig.  22. — Oxidizing  Shed. 

A,  Scrims  in  rack.    B,  Flooding  troughs.    C,  Distributing  trough. 
D,  Oil  pipe.    E,  Pump.    F,  Gutter.    G,  Oil  well. 


in  two  stages,  the  former  giving  the  name  to  the  process. 
The  shower-bath,''  as  its  nam^e  implies,  consists  of  a  large 
rectangular  box-like  chamber,  an  open-topped  perforated- 
bottomed  tank  serving  as  a  roof.  The  bottom  part  of  the 
box  serves  as  a  receptacle  for  the  oil,  10  tons  being  the  usual 
quantity  treated  at  a  time  (Fig.  23).  The  oil,  containing 
an  insoluble  drier  in  suspension  (usually  manganese  borate), 
is  run  into  the  lower  part  of  the  shower-bath,  its  temperature 
being  raised  to  90 F.  or  thereabouts  by  steam  pipes  lining 
the  sides.  It  is  then  continuously  pumped  over  the  perfor- 
ated roof  by  means  of  a  rotary  pump,  the  oil  thereby  falling 
into  the  chamber  underneath  in  a  continual  rain.     A  fan 


LINOLEUM,  FLOORCLOTH,  CORK  CARPET  167 

serves  to  change  the  air  in  the  body  of  the  oxidizing  chamber 
as  oxygen  becomes  absorbed  and  volatile  oxidation  products 
are  formed.  The  shower-bath  is  kept  in  operation  until 
such  time  (usually  60-100  hours)  as  the  oil  becomes  too 
thick  to  permit  of  efficient  subdivision  of  the  oil  into 
streams.  A  graphical  representation  of  the  progress  of 
oxidation  in  the  shower-bath  is  shown  in  Fig.  24.  The 
continuation  of  the  oxida- 
tion process  is  then  pro- 
ceeded with  in  another 
apparatus  known  as  a 
' '  smacker. ' '  The  smacker 
(Fig.  25)  consists  of  a 
horizontal  jacketed  drum 
fitted  internally  with 
rotatory  radial  arms,  of  a 
capacity  of  about  four 
hundred  gallons.  The 
thick  oil  is  run  into  the 
smacker,  the  stirring  gear 
started,  steam  passed  into 
the  jacket  until  the  tem- 
perature of  the  oil  reaches 
120*^  F.,  and  common 
whiting  added  to  the 
charge  to  an  amount  of  5 
or  6  per  cent,  of  the  oil. 
A  device  for  circulating 
fresh  air  into  the  smacker 
is  fitted  to  the  body. 
Once  started,  oxidation  in 
the  smacker  proceeds  so  rapidl}^  that  steam  is  shut  off 
from  the  jacket  and  cold  water  passes  in,  in  order  that  the 
temperature  may  not  reach  above  120°  F.  Test  cocks  at 
the  bottom  of  the  smacker  allow  of  samples  being  withdrawn 
at  intervals.  In  about  20-40  hours  the  oil  attains  such  a 
degree  of  oxidation  that  on  cooling  it  will  solidify  to  a  pale 
yellow,  rancid-smelling  solid  of  about  the  consistency  of 


Fig.  23. — Walton  "  Shower-bath." 

A  and  C,  Oil  tanks. 
B,  Oxidizing  zone. 

D,  Oil  suction  pipe. 

E,  Fresh  air  pipe. 

F,  Outlet  for  gas. 

G,  Oil  circulating  pump. 

H,  Oil  cooler. 


i68    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


common  strong  size.  The  smacked  oil also  possesses  a 
slightly  honeycombed  structure  and  is  almost  devoid  of  greasi- 
ness.  The  honeycombed  structure  is  then  developed  and  the 
last  traces  of  greasiness  removed  by  warming  to  about  160"^  F. 
in  steam-heated  ovens  for  a  few  days.  The  effect  of  the 
whiting  is  not  very  clearly  understood,  but  it  is  possible  that 
it  serves  the  twofold  purpose  of  forming  a  calcium  soap  of 
the  lower  fatty  acids  formed  by  oxidation  and  thereby 


Of 30 


/a  ^4  36  V#  60  66 

Fig.  24. 

diminishing  the  tendency  to  undue  development  of  heat  by 
reason  of  the  inhibiting  effect  of  such  soap,  and  also  neutral- 
izing the  fatty  acids  formed  which  would  otherwise  cause  the 
finished  oil  to  be  too  greasy.  This  neutralization  would  also 
result  in  the  evolution  of  carbonic  acid,  thereby  causing  the 
honeycombed  structure  in  the  smacked  oil.  From  this 
point  onward  in  linoleum  manufacture,  the  scrim  and 
the  Walton    shower-bath  "  processes  are  identical. 


LINOLEUM,  FLOORCLOTH,  CORK  CARPET  169 

Neither  ''scrim  oil"  nor  ''smacked  oil"  at  this  stage 
possesses  the  necessary  physical  properties  to  permit  of  their 
being  used  as  a  binding  medium.  Thc}^  both  possess  a 
certain  degree  of  elasticity,  but  lack  that  adhesive  quality 
or  binding  power  necessary  to  hold  together  the  other 
ingredients  used,  viz.  cork  dust,  wood  flour,  and  pigment. 
To  form  a  medium  for  amalgamation  of  these  materials  it 
is  necessary  to  obtain  a  product  of  properties  very  similar 
to  un vulcanized  rubber  in  its  warm  "  tacky  "  stage.  To  this 
end  the  oxidized  oil  requires  to  be  converted  into  "  cement." 

To  understand  the  "  cementing "  process  we  must 
first  of  all  refer  to  certain 
properties  of  the  oxidized 
oil.  If  a  piece  of  either 
scrim  or  smacked  oil  be 
cautiously  heated  to  a  tem- 
perature of  ISO""  C,  the 
solid  oil  will  melt  to  a 
thick  buttery  liquid  with 
evolution  of  considerable 
pungent  acrid  -  smelling 
fumes.  On  maintaining 
the  temperature,  the  evolu- 
tion of  gas  will  cause  the 
melt  to  rise,  the  pale  yellow 
colour  to  deepen  to  dark 
brown,  and  finally  the  melt- 
ing-point is  raised  to  such  a  point  that,  as  effervescence  dimin- 
ishes, the  mass  will  be  converted  into  a  tough  dark-brown 
elastic  solid  of  very  similar  properties  to  raw  indiarubber. 
In  this  latter  stage  the  oil  is  in  its  optimum  condition  to 
serve  as  a  binder  for  the  other  ingredients.  The  addition 
of  other  substances  during  the  cementing  process  is,  however, 
carried  out  in  practice.* 

The  cementing  process  is  carried  out  as  follows.  The 
cement  pan  (Fig.  26)  consists  of  a  steam-jacketed  cast-iron 

*  For  the  mechanism  of  the  cementing  process  and  the  effect  of  added 
substances  see  A.  de  Waele,  Joiiy.  Ind.  and  Eng.  Chem.,  191^7,  9,  6. 


Fig.  25. — Walton  "  Smacker.'* 

A,  Beaters. 

B,  Water  jacket, 

C,  Air  inlet  fan. 

D,  Air  outlet  pipe. 

E,  Oil  charge  valve. 

F,  Oil  discharge  valve. 


170    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


pan  with  a  capacity  of  200-400  gallons,  fitted  with  either  a 
vertical  or  horizontal  stirring  gear  and  a  sluice  valve  at  the 
bottom.  At  the  top  of  the  pan  is  an  opening  about  twelve 
inches  in  diameter.  In  the  case  of  the  shower-bath  process, 
the  smacked  oil  is  added  in  large  pieces  weighing  about 
fourteen  pounds  each,  whilst  in  the  scrim  process  it  is 
considered  advantageous  to  grind  the  ''skins"  to  a  meal 
through  steel  rolls  in  order  to  disintegrate  the  contained 
cotton  fabric.  Steam  is  passed  into  the  jacket,  the  stirrers 
slowly  started,  and  when  incipient  fusion  of  the  oil  takes 
place,  about  12  per  cent,  (of  the  weight  of  oil)  of  common 

rosin  in  the  molten  condition 
is  added  to  the  oil,  followed  by 
a  similar  amount  of  kauri  gum 
dust.  The  changes  detailed  above 
take  place  as  reaction  proceeds, 
and  excessive  rising  is  checked  by 
increasing  the  speed  of  the 
stirrers.  As  bodying-up,''  i.e* 
increase  of  melting-point,  takes 
place,  the  workman  in  charge  of 
the  process  judges  the  point  at 
which  to  discharge  the  contents 
of  the  pan  and  thereby  arrest  re- 
action. Towards  the  end  of  the  pro- 
cess, reaction  proceeds  very  rapidly 
and  some  considerable  experience 
on  the  part  of  the  charge  hand  is  needed,  as  both  under-  and 
over-cementing  are  equally  fatal  to  the  production  of  a 
successful  cement.  A  prompt  arrest  of  the  reaction  is 
facilitated  by  either  tipping  the  contents  of  the  cement  pan 
on  to  the  apex  of  a  mound  on  a  concrete  floor  or  preferably 
by  discharging  the  contents  into  a  hopper  leading  to  water- 
cooled  rolls  and  thence  into  separate  trays  where  cooling 
takes  place  comparatively  rapidly. 

The  preparation  of  the  sheet  consists  subsequently  in 
mechanical  treatments,  the  variations  of  which  are  adapted 
to  the  several  varieties  or  patterns  required.      Plain " 


r—  III  •  S 

Y////////////////M//m 

1 

=a  [T 

Fig.  26. — Diagrammatic  Re- 
presentation of  a  Cement 
Pan. 

A,  Charging  hole. 

B,  Discharging  valve. 

C,  Steam  jacket. 


( 


LINOLEUM,  FLOORCLOTH,  CORK  CARPET  171 


linoleum  is  the  variety  which  demands  the  simplest  treat- 
ment and  indeed  represents  in  its  mode  of  manufacture  the 
preliminary  stage  in  the  manufacture  of  all  the  varieties. 

The  amalgamation  of  the  cement,  cork  dust  (with  or 
without  wood  flour),  pigments,  and  filler  (whiting),  is  carried 
out  in  steam-heated  mixers,  the  final  formation  of  the  sheet 
being  obtained  by  passing  the  well-mixed  compound  through 
a  pair  of  rollers  slowly  revolving  at  different  speeds.  The 
thickness  of  the  sheets  obtained  is  determined  by  adjusting 
the  set or  distance  apart  of  the  rollers.  The  rolled  sheet 
is  detached  from  the  rollers  by  a  doctor  or  scraper.  One 
roll  is  internally  steam-heated  whilst  the  other  is  water- 
cooled,  this  arrangement  resulting  in  a  polished  face  to  the 
linoleum  being  obtained  and  facilitating  its  easy  detach- 
ment by  the  ''doctor.''  In  the  case  of  plain  linoleum,  the 
linoleum  mass  is  fed  between  the  rolls  with  the  jute 
or  canvas  support,  so  that  the  material  leaves  the  rolls  on 
this  support. 

The  remaining  treatment  of  the  cloth  "  is  the  same 
whichever  variety  is  manufactured.  The  canvas  backing  on 
the  green  or  unmatured  cloth  is  next  coated  with  a  lay  er 
of  a  cheap  yellow  ochre  or  red  oxide  paint  backing.  The  cloth 
travels  canvas  side  upwards  over  rollers,  and  in  a  nearty 
horizontal  position  receives  in  its  passage  a  pool  of  paint, 
which  is  spread  as  evenly  as  possible  b}^  a  trowel,  the  excess 
being  removed  as  it  leaves  the  backing  machine  by  means 
of  a  hot  roller  or  doctor.''  The  composition  of  the  paint  is 
such  that  it  requires  melting  in  a  steam-heated  pan  for  use 
and  solidifies  to  a  solid  non-tacky  coating  on  cooling  on  the 
canvas. 

The  final  process  consists  in  maturing  the  green  cloth 
in  large  buildings  internally  heated  by  steam  pipes  to  a 
temperature  of  140°-! 60°  F.  until  deemed  to  be  of  sufficient 
hardness  to  withstand  wearing,  after  which  the  rough 
edges  are  trimmed  off  so  that  the  cloth  is  of  standard 
width. 

In  printed  linoleum  the  green  cloth  is  painted  on 
its  face  by  either  machine  or  hand.    In  either  case  the  paint 


172    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


is  applied  by  means  of  a  wooden  block  toothed  in  parallel 
ribs  about  fourteen  to  the  inch,  its  outline  being  cut  to  the 
shape  desired.  The  paint  is  picked  up  from  a  pad  and  trans- 
ferred to  the  linoleum  by  pressure,  the  properties  of  the  paint 
being  such  that  a  perfect  surface  results  with  the  obliteration 
of  rib  marks  without  undue  flowing  occurring  after  the  cloth 
has  left  the  printer,  and  is  hung  up  for  maturing  in  a  vertical 
position.  In  hand  printing,  it  is  the  practice  to  apply 
successively  the  different  colours  forming  the  pattern  in  the 
portion  of  the  cloth  under  printing  treatment. 

There  are  several  varieties  of  inlaid  linoleum  made, 
the  oldest  consisting  in  building  up  the  multi-coloured  pattern 
from  green  linoleum  "  in  a  condition  of  meal  or  granules. 
A  grid,  cast  to  a  pattern  corresponding  to  the  design  required, 
is  laid  on  a  table  in  a  horizontal  position,  successive  stencil 
plates  having  openings  corresponding  to  sections  in  the  grid 
being  placed  on  top,  and  meal of  a  particular  colour 
chosen  to  fill  such  sections  is  dusted  in.  In  a  red,  white,  and 
blue  pattern,  three  stencil  plates  would  be  required.  The 
arrangement  of  the  pattern  being  conducted  on  a  sheet  of 
strong  greased  paper,  the  stencil  is  removed,  the  grid  care- 
fully lifted  so  as  not  to  disturb  the  loose  meal,''  and  the 
paper  support  with  its  layer  of  meal  pulled  along  under  a 
hydraulic  press,  the  upper  surface  receiving  a  section  of  the 
run  of  canvas  destined  for  its  final  support  (Figs.  27  and  28). 
It  will  thus  be  seen  that  this  linoleum  is  built  up  from  the 
back.  After  pressing,  the  linoleum  is  ready  to  receive  its 
backing,  after  which  the  maturing  is  carried  out  in  the  same 
way  as  in  the  case  of  printed  linoleum. 

Inlaid  linoleum  of  carpet,''  or  ragged-edged  pattern, 
is  carried  out  in  a  very  similar  manner,  the  pleasing  ragged 
outline  of  the  colour  units  being  obtained  by  dispensing  with 
the  grid  and  the  building  up  being  carried  out  from  the  face 
instead  of  from  the  back. 

The  newer  Walton  inlaids,"  characterized  by  the  sharp 
clearness  of  outline  and  greater  cohesion  of  the  finished 
product,  can  only  be  manufactured  from  oil  made  by  the 
shower-bath  process,  owing  to  the  greater  cohesion  required 


LINOLEUM,  FLOORCLOTH,  CORK  CARPET  173 
in  the  intermediate  stages  of  manufacture.    In  this  system 


c  J 


Fig.  27. — "  Granular     Inlaying  Process. 

A,  Showing  position  of  stencil  (2  colour)  on  grid. 

B,  ist  operation  showing  inlaying  of  colour  No.  i  (section  through  a,h). 

C,  2nd  operation  showing  inlaying  of  colour  No.  2. 

D,  Inlaying  operation  finished,  stencil  and  grid  removed  ;  canvas 

laid  ;  ready  for  press. 


Fig.  28. — Granular"  Inlaying  Process.    Arrangement  of  inlaying  table 

with  press. 

A,  Roller  carrying  canvas. 

B,  Feed  and  collecting  rolls  for  waxed  paper. 

C,  Roller  carrying  finished  linoleum. 

D,  Space  for  inlaying  operation. 


of  manufacture,  sheets  of  the  different  colours  required  are 
manufactured  without  the  canvas  support.    The  sheets 


174    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


are  then  led  over  rolls  fitted  with  a  knife-edged  grid,  which 
serves  to  subdivide  the  sheet  into  pieces  corresponding  in 
outline  to  the  pattern  desired  solo  pattern  or  into 
smaller  units,  the  subsequent  reassemblage  of  which  into 
local  areas  similarly  serves  to  build  up  the  desired  pattern 
(''universal  type'').  The  selection  of  particular  units  or 
assembled  units  for  the  pattern  and  rejection  of  undesired 
parts  is  accomplished  by  means  of  pistons  forming  the  bases 
of  the  knife-edged  grids,  the  pattern  being  so  built  up  that 
the  units  or  areas  required  only  are  retained  within  the 
hollows  of  the  grids,  the  undesired  units  being  pushed  out 
by  actuation  of  the  spring-carrying  piston  (Fig.  29).  After 
rejection  of  undesired  units,  the  grid-carrying  roller  contain- 
ing the  desired  units  of  sheet  required  is  pressed  into  contact 
with  a  large  cylinder  carrying  short  pins  on  its  periphery, 
on  to  which  the  units  forming  that  particular  part  of  the 
pattern  corresponding  to  the  colour  of  the  sheet  under  treat- 
ment are  transferred.  The  pinned  roller,  however,  carries 
also  the  canvas  for  the  final  support  of  the  linoleum,  so  that 
after  pressing  the  pinned  roller  carrying  its  canvas  and  ad- 
hering units  into  contact  with  a  pressure  roller,  the  finished 
linoleum  merely  requires  a  final  facing  before  maturing.  It 
is  obvious  that  the  number  of  colours  forming  the  pattern 
will  be  limited  by  the  number  of  differently  coloured  sheets 
simultaneously  under  treatment.  A  diagrammatic  repre- 
sentation of  the  machine  shown  in  the  figure  will  make  the 
process  clear. 

Cork  Carpet. — The  peculiar  resiliency  of  this  product 
necessitates  the  employment  of  a  binding  medium  of  greater 
elasticity  than  that  used  in  linoleum.  To  this  end,  oil  is 
prepared  in  a  solidified  condition  ready  to  serve  as  a  binding 
medium  without  conversion  into  cement,  the  use  of  solidi- 
fied oil  without  resins  serving  further  to  ensure  greater 
elasticity  and  resilienc}^ 

The  process  employed  for  the  solidification  of  the  oil 
is  that  known  as  the  Taylor-Parnacott  or  ''  Corticine 
process.    The  plant  consists  of  an  installation  of  six  iron 
pots  set  in  a  row  in  a  brick  furnace.    One  of  the  pots  has  a 


LINOLEUM,  FLOORCLOTH,  CORK  CARPET  175 

capacity  of  2|-3  tons,  whilst  the  other  five  are  of  half  a  ton 
capacity  each.  The  pots  are  heated  from  furnaces  on  the 
outside  of  the  building  in  order  to  minimize  fire  risk.  The 


Fig.  29. — Conventional  Representation  of  Walton  Machine  Inlaying 
2 -colour  pattern. 

A,  ''Cutting  Roller." 

B,  Roller  carrying  knife-edged  grid. 

C,  ''Pattern"  roller. 

D,  Pinned  roller. 

E,  Uncut  sheet  of  No.  i  colour, 

F,  Cutting  operation. 

G,  Cut  sheet  of  No.  i  colour. 

H,  Knife-edged  grid  outlined  to  pattern. 

J,  Tappet  on  pattern  roller  operating  discharge  of  K,  Rejected  units. 
L,  Operation  of  arrangement  of  units  of  No.  i  colour  on  pinned  roller 

carrying  selected  units  of  No.  2  colour. 
M,  Finished  sheet  ready  for  facing. 

fire  grates  consist  of  flat  trucks  mounted  on  wheels  and 
running  on  a  short  tramwa}' ,  whereby  temperature  may  be 
quickly  reduced  by  withdrawing  the  grate  on  its  rails  away 
from  the  flue  inlet. 


176    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 

A  charge  of  2^  tons  of  (preferably)  Baltic  oil  is  run  into 
the  larger  pot,  the  temperature  raised  to  about  220°  C, 
and  driers,  in  the  proportion  of  about  i  per  cent,  each  of 
litharge  and  lead  acetate,  added.  The  function  of  the  latter 
drier  is  probably  to  redissolve  any  metallic  lead  resulting 
from  the  reduction  of  the  litharge.  As  soon  as  the  driers  are 
dissolved,  a  long  iron  pipe  reaching  to  the  bottom  of  the 
pot  is  fitted,  and  a  vigorous  current  of  air  passed  into  the 
oil  by  means  of  an  air-pump.  The  fire  is  then  withdrawn, 
and  blowing  is  continued  for  several  hours.  When  moder- 
ately cool,  the  contents  of  the  pot  are  pumped  out  and 
divided  equally  among  the  five  half-ton  pots.  The  oil 
in  these  is  then  raised  to  about  250°  C.  and  maintained 
thereat.  After  six  or  seven  hours'  heating,  when  signs  of 
incipient  solidification  show  themselves,  the  oil  is  well 
stirred  and  the  fire  withdrawn.  Complete  solidification 
then  takes  place  somewhat  rapidly,  and  as  it  proceeds 
throughout  the  pot,  disengagement  of  gas  within  the  mass 
results  in  the  contents  of  the  pot  rising  to  such  a  point  that  a 
series  of  short  cylindrical  rings  have  to  be  placed  round  the 
upper  edge  of  the  pot  to  retain  the  contents  as  they  rise. 
Rising  to  a  loaf-like  head  then  ensues  and  continues  several 
hours,  after  which  a  slight  contraction  occurs.  When 
sufficiently  cool,  the  solid  oil  is  cut  away  from  the  sides  with 
a  large  hay-knife,  the  centre  cut  across,  and  a  jet  of  water 
run  into  the  bottom  of  the  pot  to  float  out  the  contents. 
The  solid  oil  then  presents  a  honeycombed  structure  in  the 
centre,  whilst  the  bottom  and  sides  are  somewhat  sticky. 

After  thorough  amalgamation  of  the  sticky  ''bottoms,'' 
dry  middles,''  and  tops  "  through  mixing  rollers,  the 
oil  is  then  ready  for  direct  use  as  a  cement  without  any 
further  treatment. 

The  further  mechanical  treatment  is  the  same  as  for 
plain  linoleum  with  the  exception  that  a  coarser  cork  is  used 
and  that,  owing  to  the  lesser  binding  power  of  the  Taylor 
oil,  proportionately  less  cork  can  be  amalgamated.  Certain 
varieties  of  cork  carpet  are,  after  maturing,  passed  under  a 
sandpapering  machine  to    buff  "  the  face  of  the  cloth.  It 


LINOLEUM,  FLOORCLOTH,  CORK  CARPET  177 


is  usual  as  a  trade  custom  not  to  apply  a  painted  backing  to 
the  canvas  of  cork  carpet. 

Floorcloth. — lyittle  description  is  needed  of  this  product, 
the  use  of  which  is  rapidly  dying  out  owing  to  the  vastly 
superior  properties  of  linoleum.  The  installation  for  the 
manufacture  of  floorcloth  consists,  in  the  case  of  the  machine- 
made  article,  of  a  pair  of  calendar  rollers  similar  to  a  backing 
machine,  where  canvas  is  coated  both  back  and  front  with 
a  coarse  paint  composed  of  linseed  oil,  linseed  oil  foots, 
varnish  foots  and  residues,  whiting,  China  clay,  and  earth 
oxides,  the  mass  being  reduced  to  a  working  consistency 
with  white  spirit,  kerosene,  or  even  water  in  certain  cases. 
The  once-coated  canvas  is  then  hung  up  in  drying  rooms  to 
harden,  after  which  the  coating  treatment  is  repeated  until 
sufficient  thickness  is  obtained.  The  final  treatment  con- 
sists in  the  application  of  a  printed  design  to  the  face. 

In  hand-made  floorcloth  the  paint  is  applied  to  the  canvas 
held  in  wooden  frames  in  a  vertical  position  by  means  of 
trowels,  the  treatment  being  precisely  the  same  as  in  the 
machine-made  article.  Hand-made  floorcloth  possesses  the 
advantage  over  the  machine-made  article  in  its  being 
capable  of  being  made  in  much  greater  widths,  six  feet  or 
two  metres  being  the  usual  limit  for  all  varieties  of  machine- 
made  covering,  whilst  eight-yard  hand-made  floorcloth  is 
commonly  made. 

Floorcloth  possesses  neither  the  wearing  properties, 
resiliency,  nor  the  heat-  and  sound-proof  qualities  of  linoleum. 

For  further  information  on  the  history  and  manufacture 
of  linoleum  the  following  papers  may  be  consulted  : — 

W.  F.  Reid,  ^^Manufacture  of  Ivinoleum,"  /.  5.  C.  /., 
1896,  jj,  75. 

H.  Ingle,  ''The  Examination  of  Linoleum  and  the  Com- 
position of  Cork,''  /.  5.  C.  /.,  1904,  1197. 

M.  W.  Jones,  ''History  and  Manufacture  of  Floorcloth 
and  Ivinoleum,''  /.  5.  C.      1919,  38,  26. 


s. 


12 


Part  V.— VARNISHES 

A  VARNISH  may  be  defined  as  a  liquid  which  dries  by  exposure 
to  the  air  to  a  more  or  less  hard,  transparent  film,  giving 
protective  action  or  decorative  effect  to  the  surface  on  which 
it  is  applied.  There  are  certain  exceptions  to  the  trans- 
parency of  varnishes,  notably  those  known  as  black  Japans, 
their  transparency  being  only  apparent  in  exceedingly 
thin  layers.  Varnishes  may  be  broadly  classified  into  three 
varieties  :  (i)  those  which  consist  of  a  medium  similar  to 
that  of  paints,  i.e,  linseed  or  other  drying  oils  and  thinner,  the 
hardness  of  which  may  or  may  not  be  increased  by  the 
addition  of  certain  other  substances  miscible  with  or  soluble 
in  the  drymg  oil  (rosin,  gum-resins,  etc.)  ;  (2)  those  which 
consist  of  a  solution  of  a  resin  in  a  volatile  solvent ;  (3)  those 
consisting  of  a  natural  product  of  a  tree,  drying  by  enzyme 
action,  i.e.  Chinese,  Japanese,  and  Burmese  lacquers. 


178 


Section  I.-OIL  VARNISHES 

ThKSE  represent  the  most  important  of  the  three  classes 
referred  to.  For  general  purposes,  oil  varnishes  may  be 
said  to  consist  of  four  constituents,  drying  oil,  resin,  a 
volatile  solvent,  and  an  accelerator  of  oxidation  of  the  drying 
oil.  These  products  therefore  dry  "  to  a  hard  film  by 
a  combination  of  evaporation  of  the  volatile  solvent  and  an 
oxidation  of  the  drying  oil  in  its  semi-liquid  combination 
with  resin.  The  application  of  a  film  or  coat  of  varnish  to  a 
surface  is  usually  done  with  a  view  to  affording  protection 
to  the  surface  against  atmospheric  destruction,  wear  and 
tear,  etc.,  there  being  exceptions,  however,  when  the  main 
object  aimed  at  is  to  confer  decorative  effect  by  giving  gloss 
and  enhancing  the  beauty  of  the  grain  of  wood.  For  either 
purpose,  however,  certain  of  the  constituents  of  oil  varnish 
have  the  same  effect.  As  stated  in  an  earlier  chapter,  lin- 
seed oil  dries  to  a  tough  elastic  film  when  subjected  to  natural 
atmospheric  oxidation.  The  oxidation  product,  Unoxyn," 
confers  the  elasticity  to  the  film,  but  neither  its  hardness, 
gloss,  nor  impermeability  is  sufficient  for  efficient  protective 
action  or  the  decorative  effect  desired.  The  resin  (the  term 
embracing  both  recent and  fossil resins)  on  the  other 
hand  is  a  body  devoid  for  all  practical  purposes  of  elasticity, 
but  of  a  very  hard  nature,  very  resistant  to  atmospheric 
effect  and  therefore  stable,  of  high  refractive  index  and 
hence  conferring  gloss.  Although  the  mixture  or  combina- 
tion of  oil  and  gum  in  a  varnish  has  not  the  same  high 
viscosity  as  it  has  after  drying  or  oxidation,  the  unoxidized 
liquid  is  yet  of  such  a  viscous  nature  that  it  would  be 
impossible  of  application  without  the  addition  of  a  diluent. 

179 


l8o    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


This  introduces  the  question  of  the  volatile  solvent.  The 
solvents  usually  employed  are  turpentine  or  white  spirit/' 
the  latter  representing  a  fraction  of  petroleum  distillate 
having  a  flash-point,  mobility,  and  rate  of  evaporation 
more  or  less  approaching  to  that  of  turpentine.  Other 
diluents  are  used,  as  turpentine  substitutes,  e.g,  shale 
naphtha,  shale  spirit,  petrol,  coal-tar  naphtha,  etc.,  but  on 
account  of  unpleasant  smell,  low  flash-point  or  other  objec- 
tionable features,  they  find  only  a  limited  application  for 
special  purposes.  The  question  of  the  employment  of 
genuine  turpentine,  or  its  most  popular  substitute  white 
spirit,''  has  occupied  the  attention  of  varnish  technologists 
to  a  considerable  degree  in  recent  years,  but  without  entering 
into  any  controversy  on  the  subject,  it  may  be  stated 
that  there  is  little  doubt  but  that  genuine  turpentine  is  the 
best  solvent  to  employ.*  Such  assertion  is  based  on  the 
facts  of  the  smaller  range  of  distillation  of  turpentine  (the 
petroleum  distillates  being  at  the  best  but  mixtures  of 
hydrocarbons  of  near  boiling-points),  its  sweeter  odour,  and 
its  property  of  absorbing  oxygen  and  thus  acting  as  an 
oxidation  catalyst  to  a  very  limited  degree.  The  most 
important  factor,  however,  is  the  superior  solvent  power  of 
turpentine  over  petroleum  spirit. f 

The  remaining  constituent  of  oil  varnishes  is  the  drier 
or  accelerator  of  oxidation.  The  mechanism  of  their  action 
has  been  referred  to  in  an  earlier  chapter,  but  it  may  be 
stated  here,  that  of  the  metals  known  to  possess  catalytic 
action  in  accelerating  oxidation,  only  the  compounds  of 
lead,  manganese,  and,  of  later  years,  cobalt,  have  found  uni- 
versal use.  J  These  driers  are  introduced  into  the  oil-gum 
combination  either  in  the  insoluble  condition  as  oxides, 
carbonates,  etc.,  or  as  metallic  salts  of  a  fatty  or  resin  acid. 
In  the  former  case,  the  high  temperature  necessary  for  the 

*  See  Part  III.,  Turpentine  Substitutes. 

I  Coal-tar  naphtha,  known  as  "  solvent  naphtha  "  in  the  trade,  has  a 
rather  greater  solvent  power  for  the  usual  constituents  of  a  varnish  than 
turpentine,  but  its  noxious  odour  and  toxicity  are  responsible  for  its  general 
inappreciation  as  a  turpentine  substitute. 

I  See  Part  II.  on  Theories  of  Driers. 


OIL  VARNISHES 


saponification  of  tlie  oil  results  in  more  or  less  darkening  of 
the  finished  product,  so  that  the  metallic     linoleates  or 
rosinates    find  favour  by  reason  of  their  solubility  at  low 
temperatures. 

From  the  foregoing  it  will  be  gathered  that  for  all  practical 
purposes  the  main  constituents  of  oil  varnish  are  the  oil 
and  the  resin.  The  simplest  type  of  varnish  would  therefore 
consist  of  a  solution  of  common  rosin  in  linseed  oil  with  the 
addition  of  a  suitable  drier  and  reduction  to  a  working 
consistency  with  turpentine.  Such  preparation  would 
indeed  constitute  a  varnish  and,  it  is  regrettable  to  state, 
has  in  the  past  found  a  certain  sale  with  unscrupulous 
dealers.  A  consideration  of  the  nature  of  the  non-volatile 
portion  of  such  preparation,  however,  will  show  the  poor 
properties  and  results  to  be  expected  from  its  use  in 
practice. 

In  the  first  place  the  mere  fact  of  the  ease  of  solution 
of  rosin  in  linseed  oil  is  such  that  amalgamation  of  the  two 
will  take  place  practically  at  normal  temperatures,  so  that 
the  linseed  oil,  having  received  no  heat  treatment  likely 
to  polymerize  its  glycerides  to  compounds  stable  to  atmo- 
spheric oxidation,  will  on  progressive  oxidation  show  all 
the  disadvantages  of  instability  earlier  referred  to.  In  addi- 
tion to  this,  it  may  be  pointed  out  that  since  polymerization 
of  linseed  oil  is  attended  by  an  increase  in  its  refractive 
index,  the  gloss  resulting  from  or  conferred  by  the  oil  con- 
stituent in  the  varnish  will  be  a  minimum.  The  other 
constituent — rosin — possesses  certain  properties  (Part  III., 
p.  96)  which  render  it  quite  unfitted  for  use  as  the  resinous 
constituent  of  a  varnish.  In  the  first  place,  it  is  decidedly 
unstable  to  atmospheric  effect,  being  reactive  to  oxidation, 
especially  in  a  thin  film,  and  its  oxidation  causes  a  loss  in  both 
weight  and  volume,  which  latter  effect  results  in  a  shrinking 
of  surface  apparent  as  cracking  of  the  film.  It  is  stated, 
moreover,*  to  yield  water-soluble  products  of  oxidation. 
The  low  melting-point,  low  refractive  index,  lack  of  toughness 

*  Paul,  Chem.  Rev.  Fett  und  Harz  Ind,,  1914,  2r,  5-8,  36-39,  53-56, 
78-80. 


i82    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


and  hardness,  are  further  properties  contributing  to  its 
general  undesirabiUty  as  a  constituent  of  varnish. 

In  a  general  consideration  of  the  product,  which  long 
experience  has  shown  to  be  composed  of  those  ingredients 
which  go  to  make  a  satisfactory  varnish,  viz.  linseed  oil, 
and  suitably  treated  fossil  gum,  it  can  be  seen  how  the 
necessary  properties  of  the  finished  varnish  are  contributed 
by  the  ingredients.  A  preliminary  word,  however,  is  here 
necessary  on  the  subject  of  the  gum-resin  or  fossil  gum.  On 
examining  a  specimen  of  a  fossil  gum  such  as  African  Anime, 
Congo  Copal,  or  Amber,  its  apparent  suitability  as  a  con- 
stituent of  varnish  is  at  once  evident  in  so  far  as  its  brilliance 
and  hardness  are  concerned.  Thus  it  would  seem  that  the 
ideal  varnish  might  be  obtained  were  the  hardness  and  lack 
of  elasticity  of  one  of  these  fossil  resins  tempered  by  the 
influence  of  linseed  oil  linoxyn.  Unfortunately  there  is  no 
means  whereby  such  a  blending  of  the  properties  of  these  two 
products  can  be  obtained,  as  the  fossil  gums  appear  to  be 
quite  insoluble  in  or  immiscible  with  linseed  oil.*  To  the  high 
degree  of  polymerization  of  the  fossil  resin  is  usually  attributed 
its  insolubility.  However,  a  solubilizing  or  depolymeriza- 
tion  of  the  gum,  resulting  in  complete  solubility  in  oil  under 
suitable  conditions,  is  obtainable  by  a  process  of  partial 
destructive  distillation,  known  among  varnish  makers  as 
running.''  In  this  treatment,  the  solubilizing  is  attended 
with  partial  decomposition  of  the  acids  to  yield  anhydrides 
or  rescues.  The  solubilized,  fused  or  run  "  gum  is,  how- 
ever, not  recognizable  with  its  original  condition,  a  consider- 
able darkening  in  colour  and  some  loss  of  hardness  having 
occurred  during  the  running  process.  The  ''run''  product, 
however,  will  have  become  by  the  treatment  soluble  in  lin- 
seed oil,  whilst  the  hardness  and  high  gloss  conferred  on  the 
gum  and  oil  combination  will  under  ordinary  conditions  of 
practice  bear  a  direct  relationship  to  those  properties  exist- 
ing in  the  natural  product. 

To  understand  the  condition  in  which  the  several  con- 
stituents exist,  a  brief  account  of  the  process  of  manufacture 

*  Kauri  gum  is  an  exception. 


OIL  VARNISHES 


183 


will  be  necessary.  A  description  of  the  preparation  of 
varnish  on  a  laboratory  scale  will  serve  as  an  introduction 
to  the  large-scale  process. 

The  first  part  of  the  process  consists  in  the  fusion  or 
running  of  the  gum.  Small  pieces  of  gum,  e.g,  Congo, 
broken  into  pieces  the  size  of  peas  are  sifted  free  from  dust 
and  introduced  into  a  250  c.c.  (tall  shape)  silica  beaker.  The 
gum  should  not  fill  more  than  one-quarter  the  capacity  of  the 
beaker  on  account  of  the  great  amount  of  frothing  which 
occurs.  The  beaker  is  suitably  supported  and  heated  over 
the  free  flame  of  a  bunsen  burner,  care  being  taken  that  the 
flame  does  not  play  above  the  height  of  the  gum  in  the  beaker. 
The  first  effect  of  the  heat  will  be  to  sinter  together  the 
individual  particles  of  the  gum,  the  whole  then  forming  a 
treacly  mass,  whilst  large  quantities  of  terpinoid  vapours 
are  evolved.  As  heating  continues,  fusion  will  extend  to 
the  rather  badly-conducting  particles  of  gum  until  the 
treacly  mass  will  have  become  converted  into  a  viscous 
liquid.  Considerable  quantities  of  gas  are  evolved,  resulting 
in  much  frothing  and  rising  of  the  contents  of  the  beaker, 
whilst  completely-fused  gum  in  the  liquid  condition  will 
begin  to  collect  as  a  liquid  darker  than  the  unfused  gum  in 
the  bottom  of  the  beaker.  The  heat  is  so  regulated  and 
vigorous  stirring  continued  that  the  maximum  rapidity  of 
complete  fusion  consistent  with  keeping  the  frothing  under 
control  is  obtained.  After  the  lapse  of  from  thirty  minutes 
to  one  hour,  according  to  the  nature  of  the  gum,  the  contents 
of  the  beaker  will  have  become  completely  fluid,  and  although 
the  temperature  throughout  the  mass  will  be  higher  than  at 
any  previous  stage  in  the  operation,  the  head  of  froth  will 
have  considerably  diminished.  At  this  point,  the  appear- 
ance of  a  sample  drawn  out  on  a  glass  rod  and  allowed  to  fall 
back  in  drops  would  indicate  to  a  practical  gum  runner  " 
the  state  of  completedness  of  the  fusion,  as  obviously  large- 
scale  practice  does  not  permit  of  inspection  through  the  walls 
of  the  metal  vessel  used.  The  gum  is  then  ready  for  the 
addition  of  the  oil.  It  is  noteworthy  to  remark  at  this 
point  that  the  evolution  of  gases  is  responsible  for  a 


i84    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


loss  of  from  15-30  per  cent,  in  weight  on  that  of  the  gum 
taken,  the  degree  of  loss  being  directly  proportional  to  the 
degree  of  fossilization  (age)  and  hardness  of  the  gum.  It 
is  also  interesting  to  note  that  there  appears  to  be  a  con- 
stant figure  for  each  class  of  gum  as  representing  the  mini- 
mum loss  in  weight  which  must  occur  before  complete 
solubility  of  the  residue  in  oil  is  obtained.  Indeed  it  has  been 
proposed  to  check  the  progress  of  the  operation  by  periodical^ 
noting  the  weight  of  the  contents  of  the  pot  during  fusion. 
Too  little  loss  results  in  partial  or  even  complete  insolubilit}' 
in  oil,  whilst  excessive  loss  results  in  loss  of  hardness  and 
darkening  of  colour  in  the  finished  product. 

The  fused  gum  is,  however,  not  soluble  in  oil  with  the 
same  facility  as  is  the  case  with  many  other  products 
possessing  the  same  range  of  solubility  in  differing  concentra- 
tions ;  thus,  if  it  be  desired  in  the  case  instanced  to  make  a 
varnish  consisting  of  equal  weights  of  gum  (unfused)  and  oil, 
the  addition  of  the  whole  of  the  oil  at  one  introduction  with 
consequent  lowering  of  the  temperature  would  induce  a 
chilling  of  the  gum  and  its  prompt  precipitation.  In  practice, 
the  oil  is  added  in  a  warm  condition,  small  portions  being 
added  until  a  test  spotted  on  to  glass  shows  a  milky  appear- 
ance on  cooling,  indicating  incomplete  union  of  gum  and  oil. 
The  temperature  is  then  raised  until  a  test  sample  shows  by 
its  clearness  on  cooling  that  complete  union  of  the  two  is 
effected,  when  a  further  addition  of  oil  is  made  and  so  on 
until  the  whole  of  the  oil  required  has  been  introduced. 

The  remainder  of  the  treatment  necessary  to  convert 
this  resin  and  oil  combination  into  a  finished  varnish  is  the 
addition  of  a  suitable  volatile  diluent  to  render  the  product 
easy  of  application,  and  the  acceleration  of  its  drying 
properties  by  introduction  of  a  drier.  The  viscous  resin- 
oil  compound  is  removed  from  the  source  of  heat  and  allow^ed 
to  cool  to  a  temperature  in  the  neighbourhood  of  the  boiling- 
point  of  the  solvent  chosen,  i.e.  160 ''-170''  C.  in  the  case  of 
American  turpentine.  Addition  of  the  thinner  is  then  made, 
care  being  taken  that  thorough  stirring  during  addition  is 
maintained  in  order  not  to  chill  locally  particles  of  the 


OIL  VARNISHES 


185 


product.  The  quantity  necessary  will  vary  and  depends 
not  only  on  the  viscosity  desired  for  the  finished  product, 
but  on  the  intrinsic  degree  of  viscosity  or  consistency  of  the 
varnish  before  addition  of  the  thinner. 

The  acceleration  of  drying  properties  in  the  product 
is  obtained,  as  already  stated,  by  addition  of  a  compound  of 
one  or  more  of  those  metals  which  are  used  to  promote  drying 
properties  in  oils,  viz.  lead,  manganese,  cobalt,  etc.  The 
introduction  of  the  same  into  varnishes  may  be  made  by 
addition  of  an  organic  compound  of  the  metal  soluble  in 
the  finished  varnish,  ix.  lead,  manganese  or  cobalt  lino- 
leates,''  rosinates,  acetates,  etc.,  to  the  product  in  the 
condition  described  above,  i.e.  after  addition  of  the  thinner, 
or  alternatively  by  causing  union  of  the  metallic  base  of  the 
metal  chosen  with  the  gum  and  oil  at  an  elevated  temperature 


Fig.  30. — Gum  Pots. 

{e.g,  200°-300°  C.)  before  addition  of  the  thinner.  In  the 
latter  case  the  base  combines  partly  with  any  free  fatty  acid 
or  resin  acid  present  and  partly  saponifies  the  neutral 
glyceride  of  the  oil. 

The  manufacture  of  oil  varnish  on  the  large  scale  differs 
in  no  material  respect  from  the  laboratory-scale  preparation 
described  above.  The  fusion,  or  running  "  of  the  gum, 
is  carried  out  in  copper  or  aluminium  vessels  of  30-100 
gallons  capacity  of  various  shapes  (Fig.  30).  The 
hearth  on  which  the  running  is  carried  out  usually  consists 
of  a  hole  in  the  floor  of  the  making  house,''  beneath 
which  is  a  grate  with  a  flue  running  underground  con- 
structed in  such  a  way  that  a  very  intense  bright  fire  may  be 
obtained  and  that  the  products  of  combustion  are  complete^ 
withdrawn  from  the  interior  of  the  making  house.  During  the 
running  "  process,  the  contents  of  the  pot  are  stirred  and 


i86    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


examined  periodically  by  means  of  a  stirrer.  The  making 
house  itself  consists  of  a  cement-floored  chamber,  well 
illuminated  and  furnished  with  as  high  a  roof  as  possible 
in  order  to  make  contingency  for  accidental  ignition  of 
the  contents  of  the  pot.  The  fumes  given  off  during  the 
running  process  are  led  either  to  a  fan  or  chimney  shaft,  or 
alternately  a  removable  cover  fitted  with  a  delivery  pipe 
leading  to  a  fan  and  condenser  installation  (Figs.  31  and  32). 

In  practice,  the  operation  of  gum-running  is  usually 
carried  out  by  two  workmen — the  gum-runner  and  his 
assistant.    A  suitable  quantity  of  gum — usually  112  lbs.  in 


Fig.  31. — Gum  Pot  with  Condenser  Installation. 

modern  practice — ^is  weighed  into  a  clean  pot  and  the  latter 
brought  to  the  fire  in  the  making  house  "  on  a  trolley  or 
truck  (Fig.  33),  occasional  stirring  being  resorted  to  during 
the  first  or  sintering  stage  of  the  fusion,  but  as  lique- 
faction of  that  portion  of  the  gum  immediately  in  contact 
with  the  bottom  of  the  pot  proceeds,  close  observation 
has  to  be  made  in  order  to  guard  against  danger  of  a 
sudden  rise  of  the  contents  of  the  pot.  The  temperature 
of  the  melt  is  regulated  by  bodily  lifting  the  pot  off 
the  fire-hole  by  means  of  the  truck.  As  the  gum  ap- 
proaches complete  fusion,  more  vigorous  stirring  is  resorted 


OIL  VARNISHES 


187 


to  until  finally  the  gum- runner  judges  the  run  accom- 
plished. The  melt  is  then  ready  for  oiling,"  i.e.  addition 
of  oil.  The  requisite  quantity  of  oil  is  added  either  by  the 
assistant  cautiously  pouring  it  in  from  a  vessel  known  as  a 
''jack/'  not  unlike  a  watering-can  with  an  old-fashioned 
kettle  spout,  or  in  more  modern  practice,  the  previously 
warmed  oil  is  pumped  up  to  an  overhead  tank  fitted  with  a 
graduated  gauge  glass  marked  off  in  warm  gallons,  from  which 
it  is  allowed  to  descend  in  a  thin  stream  into  the  gum  pot, 
the  contents  being  meanwhile  vigorously  stirred.  When 


Fig.  32.— Varnish  Furnace  In-  Fig.  33.— Varnish  Pot  with  Truck, 

stallation  with  Hood  to  remove 
fumes. 


thorough  amalgamation  of  gum  and  oil  has  taken  place,  the 
gum-oil  mixture  is  ready  for  addition  of  the  driers. 

The  rules  governing  the  addition  of  the  driers  to  linseed 
oil  apply  equally  in  the  case  of  varnish,  so  that  the  varnish 
maker  has  at  his  disposal  the  choice  of  several  metaUic 
compounds  in  different  forms  of  solubility,  etc.,  in  order  to 
produce  the  effect  he  aims  at  in  the  finished  product.  To 
recapitulate,  however,  we  may  summarize  the  common  metals 
used  as  driers  in  their  various  forms  as  follows  : — 

Class  I.  driers    . .    Lead,  iron. 

Class  II.  driers    . .    Manganese,  cobalt. 


i88    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


lyead  and  manganese  are  usually  added  in  the  form  of 
oxides,  cobalt  being  added  more  conveniently  in  the  soluble 
condition  of  rosinate,  linoleate/'  etc.  The  use  of  iron  as  a 
drier  is  considerably  restricted  by  the  dark  colour  of  its 
compounds,  but  when  introduced  is  usually  added  as  umber 
(iron  and  manganese  oxides)  or  Prussian  blue. 

The  distinguishing  characteristic  of  lead  when  introduced 
into  a  varnish  is  its  property  of  forming  an  insoluble  fraction 
(lead  stearate,  palmitate  and  possibly  oleate)  which  becomes 
apparent  after  cooling  as  a  cloudiness  which  subsequenth^ 
settles  out  as  a  precipitate  after  a  period  of  weeks  to  months. 
Rosinate  of  lead  has  less  tendency  to  cause  precipitation,  and 
when  this  occurs,  settlement  of  the  foot is  usually  fairly 
rapid,  but  the  large  proportion  of  lead  rosinate  necessary  by 
reason  of  its  low  concentration  in  lead  militates  against  its 
more  general  use  by  reputable  varnish  makers.  This  applies 
in  general  to  all  rosinates. 

Manganese  and  cobalt  have  very  similar  properties  in 
their  effect  on  varnishes  in  so  far  that  they  have  a  tendency 
to  cause  surface  drying  or  skinning  in  place  of  a  drying  right 
through  the  thickness  of  the  film.  (See  imder  Defects  of 
Varnishes"  at  end  of  chapter.)  Both  these  metals  require 
a  very  small  proportion  to  accelerate  considerably  drying 
of  the  varnish.  The  combination  of  lead-cobalt  or  lead- 
manganese  is  usually  resorted  to  excepting  where  the  pre- 
sence of  the  former  is  undesirable.  The  use  of  lead  alone  in  a 
varnish  is  not  common  owing  to  its  rather  slow  action  in 
this  form. 

In  addition  to  the  method  of  adding  driers  to  the  gum- 
oil  combination,  yet  another  method  has  found  a  certain 
amount  of  favour  with  some  manufacturers.  This  consists 
in  the  addition  of  insoluble  driers  to  the  finished  (thinned) 
varnish,  solution  being  effected  by  long  agitation  in  large 
drums.  This  method,  known  as  drumming  or  churn- 
ing,'' has  the  advantage  of  yielding  considerably  paler 
products,  probably  by  reason  of  the  sensitiveness  of  the 
metallic  soap  forming  the  drier  to  darkening  at  the  somewhat 
elevated  temperature  of  solution  obtaining  in  the  previously 


OIL  VARNISHES  189 

described  method.  The  agitation  with  contained  air  in  the 
drum  to  which  the  varnish  is  subjected  also  induces  a 
certain  amount  of  bleaching  action.  Drummed  varnishes 
have  also  the  advantage  of  a  lesser  tendency  to  skin 
over  when  stored  in  bulk  in  open  tanks.  Drummed  " 
varnishes,  however,  require  considerably  longer  periods  for 
clearing  than  varnishes  in  which  the  drier  has  been  intro- 
duced at  a  high  temperature. 

After  introduction  of  the  drier,  the  varnish  pot  is  removed 
from  the  fire  and  taken  either  to  a  cooling  room  or  to  the 
open  air,  where,  after  the  contents  have  cooled  to  a  tempera- 
ture of  i6o°-i70°  C,  addition  of  the  thinner  is  made. 
This  operation  requires  little  description,  the  turpentine  or 
particular  substitute  chosen  being  added  slowly  with  con- 
tinual agitation  of  the  contents  of  the  pot.  Addition  at 
too  low  a  temperature  may  cause  precipitation  of  the  gum 
in  certain  cases,  whilst  addition  at  too  high  a  temperature 
will  obviously  occasion  losses  owing  to  volatilization. 

The  varnish  prepared  as  described  will,  however,  not 
possess  the  necessary  properties  which  constitute  what  the 
skilled  user  would  refer  to  as  a  satisfactory  product.  In 
the  first  place,  the  varnish  will  hold  in  suspension  the 
debris  of  the  gum-bark,  mineral  matter,  etc.,  in  addition 
to  which  a  heavy  cloudiness  will  develop  a  few  hours  after 
the  varnish  is  quite  cold.  This  latter  turbidity  originates  from 
the  same  cause  as  that  accruing  from  ordinary  boiled  linseed 
oil  prepared  with  a  lead  drier,  and  is  caused  by  the  precipi- 
tation of  insoluble  lead  soap  of  the  saturated  fatty  acid  con- 
stituent of  the  oil — e,g,  stearic,  palmitic,  and  myristic  acids. 
On  long-continued  storage,  however,  both  the  debris  from 
the  gum  and  the  lead  foot will  settle  out,  leaving  a  clear 
supernatant  layer  of  varnish.  Simultaneously  with  the 
deposition  of  foots,''  other  and  more  obscure  changes  will 
occur,  chief  amongst  which  will  be  a  slight  bleaching  of  the 
varnish.  Certain  undefined  physical  changes — probably 
connected  with  alteration  in  the  arrangement  of  the  disperse 
phase  in  the  colloidal  solution — will  also  occur  on  ageing/' 
e.g.  an  improvement  in  the  flow  under  the  brush.  That 


190   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


such  change  can  be  observed  and  collated  with  changes  of 
viscosity  has  been  mentioned  in  a  paper  by  Seaton,  Probeck, 
and  Sawyer  (/.  Ind.  and  Eng.  Chem.,  1917,  p,  35).  It  is 
certain,  however,  that  the  more  reputable  houses  manu- 
facturing varnish  do  not  put  their  product  on  the  market 
until  it  has  been  matured  by  tanking  for  a  period  of  from 
six  months  to  two  years. 

In  connection  with  the  general  properties  of  the  finished 
varnish,  the  influence  of  the  particular  volatile  solvent 
employed  for  thinning  should  be  considered.  At  first 
sight,  it  might  appear  that  little  if  any  influence  would  accrue 
from  this  source,  since  the  presence  of  such  in  the  varnish 
film  is  transitory.  Considerable  discussion  as  to  the  relative 
merits  of  turpentine  and  its  more  popular  substitute  white 
spirit has  taken  place  of  recent  years  in  technical  circles, 
but  it  would  appear  that  such  has  been  mainly  concerned 
with  questions  of  odour  and  relative  toxicity  of  the  vapours 
evolved  from  the  substitute  during  drying.  The  question  of 
odour  and  toxicity,  indeed,  has  been  largely  responsible  for 
the  small  popularity  of  the  higher  fractions  of  spirit  distilled 
from  coal-tar,  e.g.  naphtha,  tar  spirit,"  etc.  From  a 
purely  physical  standpoint  it  must  be  stated  in  justice  to 
white  spirit  that  from  a  point  of  view  of  flash-point,  rate  of 
evaporation,  range  of  distillation,  etc.,  it  compares  very 
favourably  with  the  considerably  more  expensive  turpentine, 
but  a  point  that  seems  to  have  escaped  criticism  is  that  of 
the  considerably  lower  solvent  power  of  the  petroleum 
distillates.  This  point  is  of  considerable  importance  when 
dealing  with  varnishes  of  the  short-oil  "  type,  i,e,  varnishes 
containing  less  than  ten  gallons  of  oil  to  the  112  lbs.  of 
gum-resin.  Such  varnishes  require  considerably  more  skill 
in  manipulation  during  thinning,  as  the  relatively  unstable 
combination  of  gum-resin  and  oil  renders  the  former  very 
liable  to  precipitation  by  careless  addition  of  the  inferior 
solvent.  The  lower  solvent  power  of  white  spirit  also  has  an 
adverse  influence  on  the  manipulation  of  the  varnish  under  the 
brush,  as  evaporation  proceeds  on  application,  the  result  being 
a  harshness  of  flow  or     pull  "  due  to  partial  precipitation. 


OIL  VARNISHES 


Varnishes  from  Tung  Oil. — In  view  of  the  function 
which  the  oil  constituent  plays  as  an  ingredient  in  varnish, 
it  is  not  surprising  that  considerable  attention  has  been 
directed  to  the  replacement  therein  of  linseed  oil  by  China 
wood  oil.  The  greater  facility  with  which  a  high  degree 
of  viscosity  can  be  obtained  by  heat-treatnient  and  the  greater 
concentration  of  glyceride  having  an  affinity  for  oxygen,  are 
properties  which  strongly  indicate  the  suitability  of  China 
wood  oil  as  a  constituent  of  varnish.  A  further  point  which 
has  not  been  touched  upon  is  the  nature  of  the  film  yielded 
by  polymerized  China  wood  oil  on  application.  Whilst 
that  in  the  case  of  polymerized  linseed  oil  never  attains  a 
degree  of  hardness  befitting  it  for  use  alone  as  a  protective 
layer  suitable  to  withstand  a  reasonable  degree  of  abrasion, 
but  requires  the  presence  of  hard  gum-resins  to  mitigate 
against  this  softness,  the  oxidation  product  of  polymerized 
China  wood  oil  yields  a  film  which  compares  very  favourably 
with  that  of  the  linseed  oil  and  gum-resin  varnishes.  In 
addition  to  this  it  is  stated  to  possess  a  lower  degree  of 
susceptibility  to  cracking  at  low  temperatures. 

From  a  consideration  of  its  behaviour  during  heat- 
treatment,  however,  the  difficulty  of  arresting  the  thickening 
at  any  desired  degree  short  of  production  of  an  insoluble 
phase,  mitigates  considerably  against  its  use  as  sole  con- 
stituent of  a  varnish.    Similarly,  its  admixture  with  gum- 
resins  is  a  matter  of  extreme  difficulty  in  view  of  the  high 
temperature  at  which  it  is  necessary  to  effect  such  incorpora- 
tion, coagulation  of  the  oil  usually  resulting.    In  view, 
however,  of  the  remarks  above  as  to  the  intrinsic  hardness  of 
the  undiluted  oxidized  China  wood  oil  film,  the  necessity  for 
the  introduction  of  what  is  ordinarily  understood  as  a  hard 
resin  is  not  so  apparent,  and  advantage  may  be  taken  of 
the  lower  temperature  of  incorporation  demanded  by  a  soft 
resin,  both  to  enhance  slightly  the  hardness  of  the  oxidized 
film  and  facilitate  the  manipulation  of  the  oil  during  heat 
treatment.    It  is  necessary  to  remark  at  this  stage  that 
numerous  processes  have  been  suggested  claiming  to  per- 
mit of  the  thickening  of  China  wood  oil  by  heat  without 


192    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


production  of  gelatinization,*  but,  generally  speaking,  the 
claims  have  not  been  substantiated,  retardation  of  coagu- 
lation being  the  most  that  these  processes  will  effect, 
whilst  many  depend  for  their  effect  on  dilution  of  the 
tung  oil  with  a  difficultly  coagulable  constituent,  the  pro- 
portion necessary  for  which  deteriorates  considerably  the 
property  of  the  film  for  which  pure  tung  oil  is  charac- 
teristic. In  this  connection  it  has  been  found  that  a 
relatively  small  addition  of  common  rosin  to  tung  oil 
permits  of  the  desired  degree  of  polymerization  being 
obtained  without  danger  of  coagulation  when  in  the  hands  of 
a  skilled  operator.  Several  advantages  are  gained  by  such 
admixture  ;  the  undue  rapidity  of  setting  (as  distinct 
from  the  attainment  of  the  maximum  increase  in  weight, 
q.v.)  is  diminished,  and  the  liability  which  a  varnish  from  pure 
tung  oil  shows  to  exhibit  wrinkles  or  webbing  during 
drying  is  overcome. 

Many  variations  in  product  are  possible  by  employing 
different  proportions  of  rosin  and  oil  and  differences  of 
behaviour  during  drying  may  be  obtained  by  varying  the 
nature  and  proportion  of  drier,  etc.  It  is  a  significant 
fact  that  in  spite  of  the  prejudice  which  the  consumer  has 
hitherto  had  in  regard  to  varnishes  containing  rosin  as  their 
sole  resinous  constituent,  varnishes  of  this  class  were  found 
by  actual  trial  over  long  periods  to  be  the  only  ones  satis- 
factory for  the  protection  of  doped  fabrics  on  aircraft, 
linseed  oil  and  gum-resin  varnishes  proving  quite  unsuitable 
on  considerations  of  permeability,  non-adhesion  to  the  glossy 
doped  surface  and  loss  of  elasticity  for  long  periods  under 
extreme  conditions  of  temperature.  Varnishes  on  a  basis 
of  tung  oil  and  rosin  have  been  in  use  for  some  time  past  in 
the  United  States,  where  their  resistance  to  chalking  "  {q.v.) 
has  formed  the  main  basis  for  their  adoption  as  varnishes 
intended  for  sub-aqueous  work. 

It  may  be  remarked  that  the  disadvantages  attendant 
on  the  acidity  conferred  by  rosin  in  regard  to  the  use  of 
these  varnishes  as    mixing    varnishes,  i.e.  those  intended 

*  German  Pats,  219715/1910;  211405/1908 


OIL  VARNISHES 


193 


for  use  with  basic  pigments,  is  overcome  by  substitution  of 
the  acid  rosin  by  hardened  rosin  "  (alkaline  earth  rosin- 
ates)  or    ester  gum    (glycerine  rosin  ester). 

The  Properties  of  the  Varnish  on  Application. — ^In 
considering  the  uses  to  which  oil  varnishes  are  put,  their 
main  field  of  application  is  that  of  the  protection  of  wood- 
work. Besides  their  uses  as  protective  coatings,  they  have 
also  an  important  function  in  the  beautifying  and  accentua- 
ting of  the  natural  grain  of  the  wood.  This  is  accounted 
for  by  the  penetration  of  the  surface  cells  of  the  wood  by 
the  varnish,  thus  producing  optical  continuity  with  a 
resultant  exaggeration  of  the  darkness  of  the  grain. 

From  a  utilitarian  standpoint,  however,  varnish  is  applied 
for  the  purpose  of  affording  a  protective  coating  against  the 
destructive  effects  of  the  atmosphere,  moisture,  and  erosion 
generally.  Thus,  it  needs  to  be  chemically  stable  to  atmo- 
spheric influence  (oxidation) ,  physically  stable  to  changes  of 
temperature  (elastic),  waterproof,  and  withal  sufficiently 
hard  to  resist  wear.  Since  it  will  be  readily  conceded  that 
in  view  of  the  extreme  tenuity  of  the  average  coat  of  varnish 
(about  in.),  more  than  one  coating  will  be  necessary 
to  give  the  desired  result,  and  further  it  will  be  apparent 
that  one  must  consider  the  necessary  properties  which  each 
coating  must  possess  to  fulfil  its  desired  function. 

In  the  first  place,  a  preliminary  coating  will  be  necessary 
to  stop  the  capillarity  of  the  surface  cells  of  the  wood  and  at 
the  same  time  set these  cells  to  a  hard  rigid  layer  to 
obtain  a  solid  foundation.  Such  effect  would  be  obtained 
by  the  use  of  a  varnish  of  fairly  low  viscosity  to  ensure 
penetration  of  the  cells,  but  having  its  gum  and  oil  con- 
stituent of  rather  high  viscosity  in  order  that  such  penetra- 
tion, once  secured,  will  not  go  on  indefinitely,  since  a  surface 
sealing  of  the  cells  is  the  only  object  aimed  at.  Rapidity 
of  drying  will  also  increase  the  latter  effect.  Such  a  result  is 
obtained  by  the  use  of  a  varnish  known  as  goldsize."  This 
product  constitutes  a  varnish  in  which  the  gum  and  oil 
combination  has  been  brought  to  a  viscosity  approaching 
solidity,  a  liberal  addition  of  volatile  solvent  then  ensuring 
s.  13 


194   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


a  low  viscosity  in  the  finished  product.  The  nature  and 
amount  of  the  driers  added  ensure  rapidity  of  drying.  Such 
a  varnish  is  of  Umited  applicabiUty,  possessing  as  it  does  little 
gloss  in  its  dried  film  owing  to  its  extreme  tenuity,  low 
weather- resistance,  and  slight  impermeability,  but  it  fulfils 
the  purposes  for  which  it  is  designed. 

Having  now  formed  a  preliminary  base  or  groundwork, 
the  succeeding  coats  which  go  to  form  the  protective  film 
proper  need  next  to  be  considered.  From  an  examination 
of  the  properties  of  the  constituents  of  an  oil  varnish  and  of 
the  effect  which  it  is  desired  to  produce,  it  is  evident  that  from 
a  decorative  standpoint  it  is  desirable  to  obtain  as  high  a 
finish  (gloss)  as  possible,  and  to  that  end  the  relatively  high 
refractive  index  of  gum-resins  will  serve  in  good  stead. 
Thus,  for  the  bodying-up  or  under-coats,  a  varnish  rich  in 
gum-resin  is  desirable.  The  relative  hardness  of  such  short- 
oil  "  varnishes  furthermore  is  advantageous  from  the  stand- 
point of  its  rapidity  of  drying  and  soHdity.  It  is  a  recognized 
point  from  experience  that  for  the  application  of  succeeding 
coats  of  varnish,  the  glossy  coat  from  the  preceding  layer 
does  not  form  a  satisfactory  key  to  the  next,  and  it  is 
the  practice  to  prepare  the  varnished  surface  to  form  a 

tooth  or  bite  by  removing  the  excess  of  gloss.  This 
is  performed  in  practice  by  the  operation  of  flatting," 
which  consists  in  either  lightly  sandpapering  or  rubbing 
down  with  a  piece  of  hard  felt,  water,  and  pumice  powder 
the  thoroughly  hard  and  dry  surface.  Such  a  prepared 
surface  is  to  the  eye  practically  devoid  of  gloss,  and  examined 
under  a  lens  exhibits  a  serrated  or  toothed  surface  which  is 
eminently  suitable  for  the  successful  application  of  a  succeed- 
ing coat.  It  is  evident  from  this  that  for  such  purpose  to 
be  successfully  accomplished  in  practice,  a  hard-drying, 
full-bodied  varnish,  rich  in  gum-resin,  is  necessary.  Such 
varnish  constitutes  an    undercoating,''     preparation,''  or 

flatting  ''  varnish.  The  relative  proportion  of  gum-resin 
to  oil  to  form  such  a  varnish  is,  however,  not  flxed,  several 
factors  contributing  to  the  hardness  of  the  dried  film,  but 
it  may  be  stated  in  general  terms  that  the  relative  proportion 


OIL  VARNISHES 


195 


of  gum-resin  to  oil  in  this  is  always  decidedly  higher 
than  in  the  varnish  for  the  finishing  coat.  A  further 
function  of  the  undercoat  is  to  act  as  an  additional  cement 
or  binder  of  the  surface  cells  of  the  wood. 

Having  obtained  a  sufficient  body  or  thickness  of  var- 
nish, the  finishing  coating  may  be  applied.  In  this  also 
many  factors  have  to  be  taken  into  account.  A  coating  is 
required  which  shall  have  a  maximum  of  gloss,  be  of  sufficient 
hardness  to  withstand  the  ordinary  usage  to  which  the 
varnished  article  is  to  be  subjected,  and  at  the  same  time 
possess  the  requisite  elasticity  to  withstand  mechanical 
strain  and  changes  of  temperature,  in  addition  to  its  being 
both  weatherproof  and  waterproof.  On  the  latter  points, 
it  is  evident  that  varnishes  intended  for  exclusive  inside  use 
need  not  possess  that  degree  of  elasticity,  weather-  and 
water-proofness  that  are  required  for  outside  varnishes. 

The  question  as  to  the  hardness  of  the  dried  film  is 
determined  by  the  proportion  and  choice  of  gum-resin  used 
in  its  manufacture.  It  is  evident  that  the  softest  of  the 
resins,  e.g,  rosin,  would  not  adequately  fulfil  such  a  condition. 
Elasticity  is  determined,  as  referred  to  previously,  by  the 
relative  amount  of  oil  present,  so  that  to  secure  the  optimum 
of  these  two  functions,  we  should  need  to  employ  in  our 
gum-oil  combination  the  minimum  of  the  hardest  gum  obtain- 
able. The  question  of  impermeability  to  water  and  resist- 
ance to  atmospheric  effect  (oxidation  beyond  the  limit 
necessary  to  yield  a  satisfactory  wearing  film)  is,  however,  a 
very  intricate  question,  and  it  is  in  the  obtaining  of  such 
results  that  constitute  the  art  or  skill  of  the  varnish  maker. 
The  use  of  China  wood  oil  has  placed  a  valuable  adjunct 
in  obtaining  impermeable  varnishes  in  the  varnish-maker's 
hands.  In  connection  with  stability  to  atmospheric  effect, 
it  is  evident  that  for  a  given  proportion  of  oil  in  any  two 
varnishes,  since  the  reaction  of  the  drying  or  oxidation  of  a 
drying  oil  differs  in  no  way  from  other  chemical  reactions, 
the  varnish  which  dries  the  more  rapidly  of  the  two 
will  first  succumb  to  destruction  by  atmospheric  effect. 
Since  within  certain  limits,  the  drying  can  be  accelerated  by 


196   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


increasing  the  proportion  of  drier,  it  is  advantageous  to 
restrict  the  amount  added  to  a  minimum  to  secure  a  reason- 
able rate  of  drying. 

When  considering  the  purpose  for  which  a  particular 
varnish  is  destined,  it  is  important  to  bear  in  mind  these 
several  contributing  effects  obtainable  from  its  constituents. 
As  an  example,  it  may  be  stated  that  a  high  degree  of 
elasticity  would  not  be  required  in  a  varnish  destined  for 
sub-aqueous  work,  whilst  a  considerable  degree  of  imper- 
meability to  water  would  be  demanded. 

Defects  of  Varnishes 

It  will  be  gathered  that  with  such  a  complex  and  deli- 
cately constituted  combination  as  copal  varnish,  great 
liability  to  defects  after  manufacture  will  be  present  unless 
control  under  very  strictly  regulated  conditions  is  present. 
Unfortunately,  the  exigencies  of  the  manufacturing  process 
and  our  present-day  lack  of  knowledge  of  the  inner  mechan- 
ism of  the  reactions  involved  are  such  that  occasional  defects 
in  varnishes  present  themselves.  In  addition  it  is  necessary 
to  state  that  many  of  the  so-called  defects  of  varnish  during 
use  may  be  traced  to  its  application  under  faulty  conditions. 

{a)  Cracking  of  the  Film,  Flaking,  etc. — ^It  may 
happen  that  some  time  after  application,  the  previously 
elastic  film  of  varnish  may  progressively  harden  and  lose  its 
elasticity  to  a  point  at  which  with  variation  in  temperature 
of  the  surrounding  atmosphere,  development  of  cracks 
takes  place.  As  indicated  earlier,  the  elasticity  of  a  varnish 
for  its  specific  purpose  of  use  must  be  caref tdly  considered 
in  that,  under  conditions  favouring  the  progressive  hardening 
of  a  varnish  film,  for  example,  by  exposure  to  the  sun's 
direct  rays,  a  varnish  of  maximum  elasticity  must  be 
employed.  The  converse,  i.e.  the  employment  of  an  elastic 
varnish  intended  for  exterior  use  in  a  situation  sheltered 
from  the  sun's  rays,  would,  of  course,  result  in  a  film  being 
obtained  of  insufficient  hardness  for  reasonable  wear.  In 
the  case  considered,  the  development  of  cracks  would  have 


OIL  VARNISHES 


197 


for  its  origin  the  presence  of  too  high  a  proportion  of  gum- 
resin  to  oil,  with  resultant  rapid  formation  of  a  film  of  great 
hardness  and  insufficient  elasticity  under  conditions  leading 
to  changes  in  volume,  i.e.  heat  and  cold.  Cracking  of  the 
film  due  to  this  cause  usuall^^  manifests  itself  without  any 
other  attendant  phenomena.  It  may  sometimes  happen, 
however,  that  some  time  after  exposure,  the  erstwhile  glossy 
film  takes  on  a  dullness,  which  when  viewed  in  bright  sun- 
light presents  the  appearance  of  an  iridescence.  This  dulling, 
when  viewed  under  magnification,  will  resolve  itself  into  a 
network  of  fine  cracks,  which  in  course  of  time  will  develop 
and  extend  until  they  become  easily  visible  to  the  naked 
eye.  Such  destruction  of  the  film  has  for  its  origin  an  oil 
medium  in  the  varnish  of  insufficient  stability  to  the  pro- 
gressive influence  of  oxidation,  i.e.  an  oil  which  has  been 
over-treated  with  driers.  The  rapid  and  progressive 
oxidation  of  the  oil  will  result  in  a  rapid  loss  in  weight 
beyond  the  point  of  maximum  increase  (c/.  Oxygen 
Absorption  of  Oils)  with  contraction  in  volume.  A  some- 
what rarer  cause  of  cracking  in  varnish  films  originates 
from  the  wrongful  application  of  the  order  of  the 
coatings,  i.e.  the  application  of  the  relatively  less  elastic 
undercoating  varnish  on  to  the  finishing  varnish.  The 
former,  possessing  an  insufficient  elasticity  to  withstand 
variations  in  atmospheric  temperatures,  restdts  in  cracking 
of  the  film,  an  occurrence  which  is  accentuated  by  the 
comparative  lack  of  solid  support  afforded  by  the  elastic 
undercoat. 

{b)  '^Cissing/' — ^This  term,  which  would  appear  to  be 
peculiar  to  the  paint  and  varnish  craft,  represents  the 
phenomenon  occurring  when  oil  is  applied  to  a  damp  surface 
or  vice  versa,  and  consists  in  the  contraction  or  retression 
of  the  film  applied  to  separate  drops  of  lesser  surface. 
It  thus  points  for  its  causation  to  the  high  interfacial 
tension  existing  between  varnish  film  applied  and  under- 
coat,  usually  dried  varnish.  Cissing  usually  occurs  a 
few  minutes  after  application  of  a  varnish  film  to  a  glossy 
undercoat  that  has  not  been  properly    flatted    by  pumice 


198    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


powder,  etc.,  to  afford  a  "  key to  the  succeeding  coat.  The 
manifestation  of  the  cissing  "  would  appear  to  be  about 
simultaneous  with  the  period  of  evaporation  of  the  bulk 
of  the  volatile  thinner.  The  cause  of  cissing on  an 
unflatted  varnished  surface  is  to  be  found  as  before  stated 
in  the  high  interfacial  tension  existing  between  the  two 
surfaces  brought  into  contact,  while  the  inhibiting  effect 
of  flatting  is  probably  accounted  for  by  the  modification  of 
the  angle  of  contact consequent  upon  the  toothing  of 
the  flatted  surface.  Cissing  may  also  be  inhibited  by 
varnishing  the  undercoat  on  the  tack,''  i,e,  before  it  is 
thoroughly  hard  and  dry,  when  more  complete  wetting  " 
takes  place  owing  to  slight  solubility  of  the  under  surface. 
Cissing may,  however,  occasionally  occur  when 
flatting of  the  undercoat  has  been  done  and  is  then 
attributable  to  the  use  of  too  new  a  varnish.  The  newness 
of  the  varnish  need  not,  however,  be  so  great  that  cloudiness 
is  present,  i.e.  that  individual  particles  be  visible.  It  is 
probable,  however,  that  microscopic  dispersed  particles 
which  deposit  on  evaporation  of  the  volatile  solvent  act  as 
nuclei  similarly  charged  to  the  bulk  of  the  medium,  and 
consequent  repulsion  round  the  nuclei  takes  place.  The 
elucidation  of  the  cause  of  "  cissing  "  is  occasionally  somewhat 
difficult,  the  sweating  through  of  an  immiscible  constituent 
from  a  previous  undercoat,  e.g.  some  bitumens  being  some- 
times to  blame.  In  very  obstinate  cases,  washing  of  the 
undercoat  with  dilute  sodium  carbonate  solution  may 
rectify  the  fault. 

(c)  '^Pinholing"  or  Pitting. — ^This  phenomenon  par- 
takes to  some  extent  of  the  same  nature  as  cissing, and  is 
consequent  on  the  deposition  of  (generally)  visible  particles 
from  an  insufficiently  matured  varnish.  It  may  sometimes 
be  due  to  nuclei  of  moisture  on  the  undercoat  or  deposited 
subsequent  to  the  application  of  the  varnish.  When  due 
to  insufficiently  matured  varnish,  the  remedy  is  obvious. 

{d)  Blooming"  and  'Chalking. These  two  faults 
are  being  treated  under  one  heading  notwithstanding  their 
lack  of  connection,  on  account  of  the  common  misappellation 


OIL  VARNISHES  199 

of  the  one  case  for  the  other.  Blooming/'  correctly 
defined,  consists  in  the  appearance  of  a  plum-like  bloom  on 
the  surface  of  a  varnish  film,  usually  a  day  or  two  after 
drying.  It  generally  occurs  in  relatively  inelastic  short- 
oil  varnishes  containing  a  low  proportion  of  oil,  i.e,  stoving, 
rubbing,  and  polishing  varnishes,  but  it  also  occurs  in  long- 
oil  varnishes  and  in  wood-oil  varnishes.  Beyond  this 
established  fact,  there  seems  to  be  little  concensus  of 
opinion  as  to  its  ultimate  cause.  Many  observers,  whilst 
agreeing  that  its  manifestation  is  restricted  to  the  above- 
mentioned  classes  of  varnishes,  go  further  in  stating  that 
the  use  of  white  spirit  to  the  exclusion  of  turpentine  in 
the  varnish  is  the  cause.  On  such  basis,  it  would  appear 
that  the  rather  poor  solvent  properties  of  white  spirit  for 
the  relatively  unstable  combination  of  gum-resin  and  oil 
in  such  short-oil varnishes  might  result  in  a  partial 
separation  of  gum  and  oil  with  consequent  slight  optical 
discontinuity  in  the  film.  It  has  also  been  suggested* 
that  blooming  is  caused  by  condensation  of  moisture 
on  the  imperfectly  dry  varnish  film.  This  suggestion  would 
appear  to  be  substantiated  by  the  fact  that  such blooming 
may  be  temporarily  removed  by  rubbing  the  varnish  film 
with  white  spirit  or  by  washing  with  water,  f  Bloom- 
ing "  may  occur  also  on  varnishes  of  all  types  when  applied 
in  a  damp  atmosphere  under  conditions  favouring  conden- 
sation of  moisture  on  the  varnished  film.  Such  may 
exist  when  excessive  moistening  of  the  floor  has  been  done 
with  a  view  to  the  laying  of  dust  during  the  operation  of 
varnishing.  The  removal  of  the  bloom  formed  is  often 
impossible,  the  only  remedy  being  to  remove  the  coating 
and  to  re-apply  under  favourable  conditions. 

Chalking/'  or  the  manifestation  of  a  whitish  trans- 
lucence  of  the  film  on  immersion  in  water,  comes  under  an 
entirely  different  heading.  Chalking "  is  a  fault  most 
marked  in  elastic  varnishes,  i,e.  those  rich  in  oil,  but  not 
restricted  to  them,  its  intensity  on  immersion  of  a  varnish 

*  Seeligman  and  Zieke,    Handbuch  der  Lack-  und  Firnis-  Industrie," 
p.  708.  -  . 
t  Loc,  cit. 


200    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


film  seeming  to  be  roughly  proportional  to  the  relative  oil 
content,  and  being  nearly  absent  in  polishing,  etc.,  varnishes. 
It  is  essentially  an  emulsion  phenomenon  with  water  as 
disperse  phase  *  and  is  inhibited  by  the  presence  of  salts 
in  the  water.  De  Waele,t  while  calling  attention  to  the 
absence  of  the  phenomenon  in  films  of  varnishes  rich  in 
China  wood  oil,  suggests  that  it  may  be  dependent  on 
the  presence  of  suspended  water  particles,  the  degree  of 
magnitude  of  the  same,  however,  being  of  an  order  below 
that  of  the  wave-length  of  light  in  this  case.  ''Chalking'' 
disappears  on  drying  off  the  once-immersed  varnish  film, 
the  film  apparently  returning  to  its  original  state,  but  it 
returns  immediately  on  re-immersion  in  water.  J  The  fault 
is  not  one  that  should  be  present  in  varnishes  intended 
for  immersion,  e,g,  boat  and  yacht  varnishes. 

{e)  Wrinkling, — ^This,  as  its  name  implies,  appears  as  a 
puckering-up  of  that  part  of  a  varnish  film  where  a  thickness 
has  collected  owing  to  flow.  With  the  exception  of  ''  short- 
oil  "  varnishes,  which  dry  rather  by  evaporation  of  solvent 
than  oxidation,  it  is  a  fault  which  will  always  appear  when 
careless  application  of  the  film  has  been  the  case.  Its  cause 
lies  in  the  fact  that  contraction  in  volume  in  an  oil  occurs 
simultaneously  with  absorption  of  oxygen,  the  increase  in 
specific  gravity  not  corresponding  to  the  weight  of  oxygen 
absorbed.  Varnishes  which  are  inclined  to  skin  over  when 
stored  in  bulk,  especially  manifest  a  strong  tendency  to 
wrinkle  on  application.  The  formation  of  skins  is  an  indica- 
tion that  in  the  case  considered,  the  second  part  of  the 
reaction : 

A  +O2  =  AO2  AO2  +  A  =2  AO 

indicating  transference  of  oxygen  from  the  surface  layer  to 
the  under-portions  of  the  film,  is  not  complete.  The  fact 
that  skinning  is  more  often  apparent  in  varnishes  in  which 
the  oil  present  exists  to  a  great  degree  in  a  polymerized  form, 

*  R.  S.  Morrell,  Br.  Assoc.  Reports  on  Colloid  Chemistry  and  its  Indus- 
trial Applications,  1920. 

t  Proc.  Oil  and  Colour  Chemists'  Assoc,  1919,  II.,  13,  106--109. 
I  R.  S.  Morrell  {loc.  cit.). 


OIL  VARNISHES 


201 


i,e.  in  a  form  in  which  the  stability  of  peroxides  is  at  a  maxi- 
mum, would  support  this.  An  alternative  hypothesis  is 
advanced  by  H.  Wolff,*  who  points  out  that  the  rate  of 
oxidation  in  a  thick  film  of  varnish  is  considerably  greater 
in  the  surface  than  in  the  body  of  the  film,  owing  to  the  non- 
penetration  of  the  (activating)  ultra-violet  rays  in  the  latter. 
Thus,  expansion  with  partial  approach  to  a  completion  of 
the  process  of  solidification  will  take  place  in  the  surface, 
independently  of  and  in  advance  of  the  under  layers.  The 
fault  is  more  pronounced  in  certain  types  of  varnishes, 
particularly  those  in  which  solidification  is  influenced  solely 
by  oxidation  (Co,  Mn)  as  distinct  from  those  favouring 
solidification  by  polymerization  (Pb,  Ce,  etc.).t  Thus, 
wrinkling  is  accentuated  in  varnishes  which  have  been 
treated  with  an  excess  of  manganese  or  cobalt  driers. 

(/)  Bubbling." — ^The  appearance  of  small  bubbles 
during  application  of  the  varnish  changing  subsequently  to 
pinholes  is  a  minor  trouble,  generally  attributable  to  too 
rapid  an  application.  Too  low  a  viscosity,  owing  to  excessive 
thinners,  favours  the  formation  of  bubbles. 

(g)  Webbing''  or  ^'Crocodile-skin/'— This  phe- 
nomenon is  very  characteristic  and  at  first  sight  somewhat 
simulates  cracks.  On  close  examination,  however,  the  dis- 
tinction becomes  apparent  as  a  puckering-up  of  the  surface, 
the  excess  area  being  taken  up  as  ridges  forming  the 
boundaries  of  the  web.''  The  fault  always  originates  in  the 
presence  of  China  wood  oil  in  the  varnish,  suitable  treatment, 
however,  stopping  the  tendency  to  webbing/'  Webbing  " 
does  not  always  manifest  itself  on  application  of  the  same 
varnish — an  absence  or  low  concentration  of  ultra-violet  rays, 
together  with  an  atmosphere  highly  charged  with  CO2,  etc., 
favouring  its  appearance.  Generally  speaking,  it  may  be 
attributed  to  the  presence  of  a  large  proportion  of  insuffi- 
ciently polymerized  China  wood  oil.  Since  many  modem 
varnishes  are  made  with  China  wood  oil  as  a  constituent, 
the  liability  to     webbing  "  should  be  specially  tested  for 

*  Farben-Zeit.,  191 9,  24,  11 19. 

t  R.  S.  Morrell,  /.  Chem.  Soc,  191 8,  113,  iii. 


202    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


by  allowing  glass  plates  coated  with  the  varnish  to  dry  in 
an  atmosphere  highly  charged  with  products  of  combustion. 

{h)  Blistering  and  Peeling. — Both  these  faults  are  attri- 
butable to  the  same  cause,  viz.  the  entrapping  of  ultimately 
volatile  products  underneath  varnish  films.  In  the  case  of 
blistering,  solid  impurities,  often  heavy  metal  soaps  (driers), 
arising  from  imperfect  maturing  of  the  varnish,  set  up  local 
points  of  decomposition  of  the  oxidized  film  immediately 
in  contact  with  the  undercoat,  with  consequent  development 
of  gaseous  products  of  decomposition  which  are  unable  to 
penetrate  the  covering  film.  The  fault  is  naturally  more 
common  where  the  varnish  film  is  exposed  to  heat.  No 
remedy  exists  when  once  the  fault  occurs,  but  careful 
flatting  of  the  undercoat  is  usually  a  safeguard  against  its 
occurrence. 

Peeling  is  similarly  less  likely  to  occur  when  thorough 
flatting  of  the  undercoat  has  been  done,  owing  to  the  more 
perfect  key  "  or  adhesion  of  the  two  surfaces.  It  is  usually 
caused  by  the  application  of  a  varnish  coating  on  an  under- 
coat containing  too  much  drier,  for  the  reasons  given  above 
as  to  the  cause  of  blisters  forming  round  nuclei  of  drier 
particles. 

{i)  **Ropiness." — RopinCvSS,  or  the  curdling  of  varnish 
under  the  brush,  although  more  apparent  during  use,  has 
usually  occurred  previous  to  its  application.  In  its  simplest 
form,  it  is  due  to  chilling  of  the  varnish  with  consequent 
disturbance  of  the  accurately  gauged  viscosity  and  flow 
originally  present.  It  must  not  be  lost  sight  of  that  copal 
varnishes  are  at  best  somewhat  unstable  emulsoids  or  sus- 
pensoids  very  susceptible  to  irreversible  change  under  extreme 
conditions  of  cold.  Curdling,  however,  is  sometimes  attri- 
butable to  faulty  manufacture,  i.e.  incomplete  solution 
of  the  gum-resin  in  the  oil.  It  manifests  itself  as  a  gradual 
increase  in  viscosity  during  storage  with  ultimate  separa- 
tion of  a  gummy  coagulum.  Many  varnishes  are  curdled 
by  addition  of  raw  or  boiled  linseed  oil,  petroleum  spirit, 
other  varnishes,  etc. 

General  Flaws  or  Imperfections  in  the  Surface  of  a 


OIL  VARNISHES 


203 


Varnish  Film  are  often  apparent  to  a  degree  out  of  all 
proportion  to  their  cause,  very  small  particles  of  falling  dust, 
etc.,  causing  small  pips,  ridges,  etc.  The  careful  straining 
of  the  varnish  previous  to  use,  employment  of  clean  brushes, 
working  in  as  far  as  possible  a  dust-free  atmosphere,  etc., 
will  do  much  to  obviate  these  imperfections. 

Types  of^  Varnishes 

A  general  description  of  the  manufacture  of  a  typical 
varnish  composed  of  gum-resin,  drying  oil,  siccative  and 
volatile  thinner  has  been  given  in  the  preceding  pages.  It 
being  beyond  the  scope  of  this  work  to  enter  into  the  composi- 
tion of  the  many  varnishes  destined  for  special  purposes, 
a  brief  description,  however,  of  the  more  general  types  will 
not  be  amiss. 

Decorator's  Varnishes. — ^These  varnishes,  as  their 
name  implies,  are  intended  for  use  by  the  house  decorator, 
in  distinction  to  the  more  exacting  requirements  of  the  coach 
or  motor  body  painter.  With  the  exception  of  Goldsize 
or  Preparation  Varnish  already  referred  to,  these  varnishes 
are  characterized  by  a  fair  degree  of  elasticity  and  moderate 
speed  of  drying.  Church  Oak  is  the  term  usually  applied 
to  a  varnish  yielding  an  especially  hard  film  which  does  not 
appreciably  soften  at  temperatures  up  to  40°  C,  it  being 
intended  for  use  on  seats.  The  hardness  of  the  film  is  often 
obtained  by  addition  of  lime  to  the  gum-resin  and  oil  combina- 
tion, a  calcium  resinate  of  high  melting-point  being  formed. 
Mixing  Varnish  is  specially  intended  for  conferring  gloss  to 
oil  paints,  and  in  consequence  of  the  liability  of  the  latter  to 
contain  basic  pigments,  should  have  as  low  an  acid  value  as 
possible.  Inside  Varnishes  are  usually  of  high  gum-resin 
content,  thus  giving  a  great  degree  of  hardness  and  gloss, 
but  not  necessarily  of  great  weather-resisting  power. 

Coach-builder's  Varnishes. — ^These  represent  the  high- 
est art  of  the  varnish  manufacturer,  and  are  characterized 
by  their  careful  preparation,  long  maturing,  special  selection 
of  gum-resins,  treatment  of  oil,  and  nature  and  proportion 


304   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


of  driers  for  which  the  particular  varnish  is  specially  destined. 
It  will  thus  be  apparent  that  none  but  specially  selected  pale 
hard  gum-resins  and  carefully  treated  oils  would  be  suitable 
for  the  pale  varnishes  known  in  the  trade  as  French  Oil, 
Maple  and  Venice  Body  Varnishes.  The  terms  Body  Varnish 
and  Carriage  Varnish  refer  to  their  intended  designations 
of  application  to  the  body  (doors,  roof) ,  and  under  portion 
(wheels,  etc.)  of  coaches.    Japans  will  be  dealt  with  below. 

Varnishes  for  Special  Purposes  include  such  types  as 
Boat  and  Yacht  Varnishes  (specially  prepared  to  withstand 
immersion  in  water) ;  Stoving  Varnishes  for  the  protection 
of  metal  parts  where  rapid  application  and  great  hardness 
are  essential ;  Polishing  Varnishes  intended  for  decoration 
of  articles  not  exposed  to  the  weather,  in  which  the  finish 
is  obtained  by  hand  polishing  the  flatted  dried  film,  etc. 

Japans  and  Bituminous  Varnishes 

These  preparations  occupy  a  special  position  to  themselves, 
since  they  differ  in  having  for  their  characteristic  ingredient 
a  bitumen  or  asphalt.  Both  gum-resin  and  oil  may  or 
may  not  be  present,  as  the  asphaltic  substances  differ  far 
more  widely  in  their  hardness  and  elasticity  than  the  various 
gum-resins,  and  correction  of  these  properties  to  the  degree 
desired  is  thereby  obtained.  The  most  important  bituminous 
varnish  used  in  the  higher  branches  of  the  decorative  crafts 
is  known  as  Black  Japan.  The  method  of  preparation, 
specific  choice  of  ingredients,  etc.,  varies  widely  with  the 
different  manufacturers,  so  that  beyond  a  general  descrip- 
tion of  the  composition  and  properties  of  this  product, 
reference  must  be  made  for  further  details  to  the  larger 
text-books.  The  specific  purpose  of  Black  Japan  is  in  the 
production  of  a  brownish-black  ground  of  particular  trans- 
lucency  or  depth,  thus  differing  from  pigmented  preparations 
(black  enamel)  which  appear  to  reflect  their  depth  of  colour 
from  the  surface  only.  The  main  use  of  Black  Japan  in 
the  coach-building  craft  is  in  the  treatment  of  mudguards, 
etc.     The*^  varnish  itself  is  obtained  by  amalgamating 


OIL  VARNISHES 


205 


specially-treated  oil  of  great  drying  power  with  a  suit- 
able bitumen  in  the  same  way  as  described  in  the  manu- 
facture of  copal  varnish.  The  Japan  itself  rarely  possesses 
a  great  degree  of  elasticity  or  weather-resistance,  so  that 
in  use  it  needs  to  receive  a  coating  of  a  suitable  finishing 
varnish.  The  art  of  the  varnish  maker  consists  in  the 
preparation  of  a  Japan  of  a  great  degree  of  depth  and 
intensity  of  colour  without  employing  so  high  a  proportion 
of  bitumen  that  solubility  of  the  latter  in  the  ensuing 
coat  takes  place,  a  condition  manifesting  itself  by  the 
appearance  on  the  finished  work  of  an  undesirable  greenish 
fluorescence.  Brunswick  Black,  Air-drying  Black,  and 
Black  Stoving  Enamels  also  belong  to  the  class  of  Black 
Japans.  Black  Varnish  for  patent  leather  is  a  varnish 
possessing  an  extraordinary  degree  of  elasticity  at  low 
temperatures  combined  with  a  high  gloss  and  great  trans- 
parency. It  is  obtained  by  suitable  heat  treatment  of 
linseed  oil  in  the  presence  of  iron  as  a  drier  or  polymeriza- 
tion accelerator,  Prussian  blue  being  commonly  employed 
as  the  source  of  iron.  Addition  of  gum-resin  or  asphaltum 
is  inadmissible  on  account  of  the  extreme  elasticity  needed. 


Section  II.-INSULATING  VARNISHES 


Owing  to  the  high  dielectric  constants  of  resins,  drying  oils, 
and  pitches,  varnishes  are  valuable  insulating  materials. 
The  following  table  shows  the  specific  inductive  capacity  of 
a  number  of  insulating  materials  : — 


The  solution  of  resin  and  oil  in  suitable  thinners  permits 
impregnation  of  other  insulating  materials,  and  after  volatili- 
zation of  the  solvent  a  film  of  non-conducting  material  is 
left  with  high  moisture-resisting  properties.  In  the  case 
of  many  varnishes  the  film  is  sufficiently  elastic  to  bear 
strain,  and  rise  of  temperature  in  working  is  often  insufficient 
to  cause  brittleness  so  that  the  surface  coating  remains  intact 
for  long  periods  of  time.  The  description  of  insulating 
varnishes  is  inadequate  in  most  text-books  on  varnishes  in 
English,  although  some  firms  issue  brochures  describing 
the  properties  and  uses  of  their  preparations.  It  is  left  to 
the  electrical  engineer  to  put  forward  his  requirements  and 
to  state  the  conditions  under  which  the  materials  are  used. 
In  this  brief  statement  of  the  subject  the  general  principles 
of  the  functions  of  insulating  materials  as  laid  down  by 
Fleming  and  Johnson  in  their  handbook,  Insulation  and 
Design  of  Electrical  Windings (lyongmans,  1913),  have  been 
closely  followed.    To  the  varnish  student  it  will  be  evident 


Asphaltum  . . 
Plate  glass 
Paraffin 
Shellac 
Resin 

Porcelain    . . 
Paper 
Mica 
Bakelite 


2-68  (Pirani) 
5 -3 7-6 -2  (Arons) 

2 -  32-1 -92  (Bergmann) 

3 -  1-2  7  (Gordon) 
2 '55  (Boltzmann) 

4-  8-6-8  (Curie) 
2*o~2*5  (Pirani) 
6*64-5-66  (Klemenic) 

5-  6-8-8  (Electric  Test  Laby.,  N.Y.) 


INSULATING  VARNISHES 


which  classes  of  varnishes  are  most  suitable  for  different 
systems  of  insulation  without  going  into  details  of  the 
composition  of  the  mixings.  To  the  electrician  a  break- 
down due  to  defective  insulation  is  costly  and  leakage  of 
current  makes  economical  and  safe  working  impossible. 
The  varnishes  of  insulating  coatings  must  give  maximum 
insulation  with  minimum  thickness  and  maximum  penetra- 
tion combined  with  marked  waterproofing  properties. 

Fleming  and  Johnson  classify  insulating  varnishes  as 
follows  : — 

(1)  Varnishes  for  impregnating  windings. 

(2)  Varnishes  for  treating  paper  and  fabrics. 

(3)  Cementing  varnishes. 

(4)  Finishing  varnishes. 

(i)  Varnishes  for  Impregnating  Windings. — ^These 
varnishes  are  used  to  fill  the  coverings  of  windings,  thereby  in- 
creasing their  insulating  value  and  rendering  them  moisture- 
resisting.  They  must,  moreover,  resist  the  action  of  the  hot 
mineral  oil  which  is  essential  for  oil-immersed  windings.  The 
film  should  be  sufficiently  flexible  to  withstand  mechanical 
stresses,  and  expansion  and  contraction  of  the  windings, 
and  retain  their  flexibility  with  age.  There  must  be  no 
corrosive  action  on  copper  or  destruction  of  the  fibrous 
insulation  coverings.  The  varnishes  should  be  as  free  as 
possible  from  organic  acids  which  tend  to  become  active 
during  the  process  of  oxidation  of  the  varnish  especially 
when  it  is  stoved.  This  corrosive  action  of  the  organic 
acids  is  not  serious  except  for  fine  windings,  i.e.  wires  of 
0*02  in.  diameter  and  smaller.  The  corrosive  action  of  the 
acids  appears  to  cease  when  the  varnish  becomes  dry. 
The  green  discoloration  often  noticed  on  windings  impreg- 
nated with  linseed  oil  resin  varnishes,  especially  those 
containing  turpentine,  occurs  during  the  drying  process,  and 
when  once  the  varnish  has  hardened  no  further  action  seems 
to  take  place.  Experience  has  shown  that  the  acids  in  var- 
nishes are  not  the  main  cause  of  insulation  failures.  The 
presence  of  a  certain  amount  of  mineral  drier  is  permitted,  if 
air-drying  coatings  are  required,  and  also  in  stoving  varnishes. 


2o8    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


It  must  be  pointed  out  that  the  oxidation  of  a  varnish 
continues  slowly  even  after  it  appears  dry,  so  that  the 
elasticity  will  gradually  decrease  and  care  must  be  taken  in 
the  amount  of  drier  present  and  in  the  proportions  of  resin 
and  oil  present  in  the  mixing. 

An  air-drying  varnish  is  usually  made  to  dry  in  10-15 
hours  and  a  stoving  varnish  in  12-15  hours  at  a  temperature 
of  100°  C. 

Pitch  varnishes  containing  linseed  oil  are  of  more  per- 
manent flexibility,  but  have  a  lower  dielectric  strength  and 
are  less  mineral-oil  resisting. 

In  linseed  oil  varnish  films  only  the  outside  is  really  dry. 

If  a  resin  dissolved  in  spirit  or  suitable  petroleum  thinner 
is  used  the  windings  are  dry  on  the  expulsion  of  the  solvent, 
but  such  films  lack  the  flexibility  of  linseed  oil  varnishes. 

The  shellac  varnishes  are  suitable  for  insulation  of  low 
voltage  windings  such  as  instrument  coils,  where  the  temper- 
ature variation  is  small  and  there  are  no  mechanical  stresses 
to  be  considered.  The  coatings  are,  however,  brittle,  and 
although  the  shellac  dries  quickly  it  is  not  permanent  in 
moist  air. 

The  so-called  heat  -  radiating  varnishes  are  compara- 
tively poor  insulators  and  more  satisfactory  results  are 
obtained  by  filling  interspaces  in  the  windings  with  solid 
compounds. 

It  is  of  the  greatest  importance  to  remove  moisture  from 
the  cotton  coverings  before  the  varnish  is  applied,  other- 
wise insulation  may  break  down  due  to  ionization  caused  by 
the  water.  The  drying  of  the  windings  is  performed  prefer- 
ably in  a  vacuum  chamber,  and  the  varnish  should  be  allowed 
to  impregnate  the  coils  prior  to  removing  them  from  the 
vacuum  chamber,  thereby  ensuring  greater  penetration  of 
the  varnish.  The  varnishes  require  thinning  from  time  to 
time  with  benzine  to  maintain  the  penetrating  power,  and 
special  care  should  be  taken  to  ensure  that  the  first  coat  is 
dried  before  a  second  coat  is  applied.  When  more  than  one 
coating  is  applied  the  windings  should  be  drained  from 
opposite  ends  to  ensure  uniformity  of  coating. 


INSULATING  VARNISHES 


Solid  impregnating  compounds  on  an  asphaltum  basis 
have  now  considerably  superseded  varnishes  in  the  insula- 
tion of  windings.  The  impregnation  is  complete  in  one 
operation,  the  coatings  are  more  chemically  inert  and  are 
better  fillers  and  more  moisture-resisting.  A  difficulty  in 
their  use  is  selection  of  the  asphaltum  which  will  impregnate 
the  windings  at  a  suitable  temperature  and  will  not  ooze 
out  of  the  windings  during  the  working.  Generally  the 
asphaltums  used  soften  at  io5''-ii5°  C.  and  do  not  become 
appreciably  fluid  below  150°  C.  If  the  impregnation  temper- 
ature is  raised  too  high  the  coverings  may  be  carbonized  and 
if  too  low  the  compound  may  ooze  out  when  the  winding  is 
in  use.  These  compounds  are  used  on  field  coils  and  station- 
ary windings,  but  not  for  revolving  parts,  owing  to  displace- 
ment by  centrifugal  forces,  nor  for  transformer  work,  owing 
to  rise  in  temperature,  or  for  immersed  types  of  transformers 
where  the  asphaltum  may  be  dissolved  away. 

Method  of  Application. — ^The  windings  to  be  impreg- 
nated are  dried  in  a  vacuum  chamber.  The  compound 
-  is  melted  in  a  steam-coil-heated  tank  and  introduced  into 
the  vacuum  chamber  by  opening  a  valve  in  the  connecting 
pipe,  the  atmospheric  pressure  forcing  the  compound 
out  of  the  melting  tank  into  the  vacuum  chamber.  The 
windings  in  the  vacuum  chamber  are  now  subjected  to 
an  air  pressure  of  50  lbs,  per  square  inch  so  as  to  complete 
the  impregnation  of  the  coils. 

(2)  Varnishes  for  Impregnating  Papers  and  Fabrics. 

— ^The  requirements  are  good  flow,  high  moisture-resisting 
power,  and  great  flexibility.  It  is  of  great  importance  that  the 
physical  properties  of  the  coating  materials  should  be  perma- 
nent under  working  conditions.  They  must  not  be  attacked 
by  mineral  acids.  The  presence  of  mineral  acids  is  immaterial 
provided  there  is  no  chemical  action  between  them  and  the 
paper  or  fabric.  Owing  to  the  natural  moisture-absorbing 
powers  of  the  paper  and  fabric  they  must  be  heated,  but  not 
too  much,  to  reduce  their  flexibility.  As  in  the  case  of  all 
instdating  varnishes  a  proper  balance  must  be  maintained 
between  stoutness  and  penetration,  and  the  selection  of  a 
s.  '  14 


210    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


variety  will  largely  depend  on  this  factor.  The  thickness 
of  the  coating  is  about  o*oo2  in.  A  linseed  oil  varnish 
can  be  employed  containing  resin  with  driers,  which  can  be 
stoved  at  ioo°  C.  to  give  a  smooth  elastic  surface  with  due 
consideration  of  the  penetrating  power  and  elasticity  of 
the  film. 

(3)  Varnishes  for  Cementing  Purposes. — ^These  var- 
nishes are  used  in  building  up  mica  sheets  and  in  the 
preparation  of  mica  cloth.  Spirit  varnishes,  shellac,  or  spirit 
soluble  copal  in  methylated  spirit  may  be  used.  These 
varnishes  dry  without  oxidation  and  soften  on  heating,  so 
that  their  use  is  restricted  by  temperature  limitations. 
They  are  employed  for  the  preparation  of  insulating  tubes 
and  for  cylinders  of  paper  and  mica. 

(4)  Varnishes  for  Finishing  Purposes. — ^These  varnishes 
are  used  to  give  a  smooth  coating  to  the  coils  or  windings 
which  will  prevent  dirt  or  dust  from  accumulating  on  the 
surface.  They  must  be  air-drying  and  resist  lubricating  oil. 
Shellac  and  pitch  varnishes  give  a  smooth  surface  but  are 
brittle.  The  varnishes  may  be  sprayed  on.  Provided  the 
windings  are  protected  by  smooth  tape  the  impregnating 
varnish  is  sufficient,  although  the  surface  must  be  hand- 
brushed,  which  adds  to  the  cost  of  production. 

For  testing  the  insulation  power  it  is  customary  to  include 
drying  tests,  breakdown  voltage,  the  effect  of  hot  mineral 
oil,  brittleness,  economy  test  (which  is  a  test  to  determine 
the  proper  concentration  of  a  varnish  giving  the  maximum 
breakdown  voltage),  shop  tests  with  the  varnish  in  bulk, 
penetration  tests  whereby  the  depth  to  which  a  varnish  has 
penetrated  the  material  built  up  in  layers,  and  finally  the 
rate  of  moisture  absorption  by  measuring  the  insulation 
resistances  at  frequent  intervals  of  a  coil  immersed  in  water. 

The  testing  of  a  varnish  as  carried  out  by  the  National 
Physical  Laboratory  would  be  on  the  lines  of  impregnating 
coils  of  double  cotton-covered  wire  and  submitting  adjacent 
layers  of  the  coils  to  an  increasing  alternating  voltage  until 
breakdown  occurs.  The  pressure  would  be  applied  to  the 
coils  in  steps  of  100  volts  for  periods  of  i  minute  starting 


INSULATING  VARNISHES 


211 


at  500  volts.  The  varnish  coating  would  be  stoved  at 
155°  F.,  23"  vacuum  or  240°  F.  at  ordinary  pressure.  A  good 
elastic  insulating  varnish  tested  under  these  conditions 
would  stand  an  electric  pressure  of  1400  volts  before  breaking 
down.  The  coils  would  be  dipped  only  once  in  the  varnish. 
Doubly  coated  the  protection  would  be  greater.  It  must 
be  pointed  out  that  direct  current  breakdown  voltages  are 
always  higher  than  alternating  (about  twice),  probably  due 
to  less  internal  heating  and  absence  of  alternating  dielectric 
stress.  Generally  the  voltage  required  to  produce  a  break- 
down should  be  about  1000  volts  per  mil.  in  a  thickness  of 
0'Oo6  in. 

A  few  figures  may  be  quoted  from  Seeligman  and  Zieke 
to  give  some  idea  of  the  breakdown  voltages,  but  they  must 
not  be  taken  as  anything  more,  as  so  much  depends  on  the 
thickness  of  the  layer,  the  penetrating  power  of  the  varnish, 
and  the  general  conditions  of  the  tests — 


In  Part  III.  reference  was  made  to  the  uses  of  Bakelite  for 
insulating  purposes.  Paper  impregnated  with  Bakelite 
varnish  is  used  in  the  manufacture  of  Micarta sheets, 
Pectinax tubes,  and  transformer  terminals.  Paper  is 
impregnated  with  the  varnish  and  dried  ;  and  a  sufficient 
number  of  the  sheets  of  paper  are  then  pressed  together  or 
rolled  into  tubes.  In  this  way  cylinders  up  to  3  ft.  in 
diameter  and  transformer  terminals  from  6-9  ft.  long  are 
made  to  stand  100,000  volts  (Williams,  Electric  World/' 
1911). 

The  dielectric  strength  of  sheets  of  impregnated  Bakelite 
paper  is  53,700  volts  for  in.,  compared  with  50,460  volts 
with  shellac  paper.  The  varnishes  may  be  used  for  the 
impregnation  of  coils,  armatures  for  magnetos,  arc  lamps, 
transformers,  etc.  Fluid  Bakelite  ''A''  can  be  used  for 
impregnation  of  arc  lamps  or  magneto  coils.    It  penetrates 


Shellac  (one  coating)    . . 
Linseed  oil  (drying  oil) 
Stand  oil 


650 
2400 
650 
400 
3000 


450  volts. 


Dammar  in  turpentine 

Flatting  varnish 

An  elastic  insulating  varnish 


212    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


through  the  windings  and  forms  a  solid  block.  It  has  not  as 
yet  replaced  asphaltum  impregnation.  The  coils  are  said, 
to  resist  mechanical  shocks  better  than  when  resins  are  used 
and  the  heat-conducting  properties,  although  weak,  are 
greater  than  those  of  ordinary  resinous  materials.  It  resists 
acids,  chlorine,  and  weak  alkalies.  It  must  be  remembered 
that  Bakelite  requires  a  stoving  heat  of  I40°-I70°  C,  and 
the  excessive  strength  of  the  filling  renders  the  dismantling 
of  the  parts  difficult. 

In  view  of  the  varied  requirements  for  an  insulating 
varnish  it  is  evident  that  special  varnishes  are  wanted  for 
different  kinds  of  work.  The  risk  in  employment  of  a  new 
kind  of  varnish  must  be  great  because  of  failure  in  insulation 
and  its  effects.  Although  much  can  be  avoided  by  careful 
preliminary  testing  it  must  be  remembered  that  a  durability 
test  takes  time  under  working  conditions.  Thus,  once 
a  variety  has  been  found  to  give  satisfactory  results  there  is 
little  inclination  to  change,  unless  inducements  are  offered 
in  the  form  of  improved  insulation,  combined  with  cheapness 
of  material  and  of  application. 


Section  III.-SPIRIT  VARNISHES 


Spirit  varnishes  are  characterized  by  their  consisting  of  an 
alcohoUc  solution  of  resin  or  mixture  of  resins,  so  that  the 
drying  which  takes  place  on  application  on  a  surface 
consists  merely  in  an  evaporation  of  the  solvent,  the  resinous 
constituent  remaining  in  its  original  state  as  before  solution  : 
they  thus  possess  the  advantage  of  drying  much  more  rapidly 
than  oil  varnishes.  Owing  to  the  fact  that  a  constituent 
which  possesses  the  same  elastic  properties  as  the  dried  oil 
or  linoxyn  in  an  oil  varnish  has  not  yet  been  found,  the  result- 
ant film  is  necessarily  more  brittle  and  never  possesses  the 
toughness  which  would  fit  it  for  the  hard  wear  to  which  the 
latter  class  of  varnishes  is  subjected.  The  properties  of  a 
spirit  film  varnish  being  intrinsically  those  of  the  particular 
resins  chosen  there  is  a  certain  variety  of  properties  in  the 
dried  film  which  are  limited  only  by  the  solubility  of  the 
resins.  Although  the  individual  resins  possess  varying 
degrees  of  toughness,  a  toughness  approaching  that  of  even 
a  moderately  elastic  oil  varnish  cannot  be  obtained.  It  may 
be  pointed  out  that  among  the  spirit  varnish  resins  may  be 
found  some  which  possess  the  properties  of  impermeability 
to  gases  and  water,  insulation,  etc.,  which  are  not  obtainable 
to  the  same  degree  in  oil  varnishes.  Some  improvements 
in  elasticity  of  spirit  varnish  films  is  obtained  by  the  addition 
of  certain  substances,  which,  however,  at  the  same  time 
seriously  diminish  their  hardness  and  wearing  properties. 
Spirit  varnishes  lend  themselves  especially  to  the  preparation 
of  coloured  varnishes  owing  to  the  ease  with  which  aniline 
colours  can  be  dissolved  in  methylated  spirit. 

Preparation. — Of  the  preparation  of  spirit  varnishes 

213 


214   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


consisting  merely  in  the  solution  of  the  resin  in  alcohol  little 
description  need  be  given.  There  are  three  methods  in 
general  use :  the  first  consisting  merely  in  stirring  the  sub- 
divided resin  in  a  suitable  quantity  of  spirit  in  an  open  vat 
until  solution  has  taken  place  and  then  allowing  the  insoluble 
impurities,  bark,  wood,  etc.,  to  separate  by  settlement  or 
filtration.  The  other  methods  purport  to  accelerate  the 
solution  of  the  resin,  the  most  widely  used  being  that  of 
accomplishing  solution  in  a  horizontally-disposed  wooden 
barrel  fitted  with  a  manhole  for  charging,  the  barrel  being 
rotated  about  its  horizontal  axis  either  on  geared  spindles  or 
on  trunnions.  The  third  method  consists  in  dissolving  the 
resin  in  spirit  in  a  closed  steam- jacketed  digester  fitted 
internally  with  stirring  gear.  Although  the  hand-stirring 
method  of  solution  appears  primitive  it  is  claimed  by  many 
authorities  that  paler  varnishes  are  obtained  by  this  means 
owing  to  the  lesser  contact  with  air  to  which  the  varnish  is 
subjected. 

The  further  treatment  of  the  freshly  prepared  spirit 
varnish,  viz.  the  clearing,  is  either  accomplished  by  simple 
sedimentation  or  filtration ;  in  the  latter  case  special 
precautions  have  to  be  taken  owing  to  the  volatility  of  the 
solvent.  Shellac  varnishes,  i.e.  alcoholic  solutions  of  gum 
shellac,  will  only  clear  to  a  limited  extent  by  sedimentation, 
a  considerable  amount  of  turbidity  being  present  even  after 
long  standing,  and  filtration  not  being  practicable  owing  to 
the  fine  subdivision  and  slimy  nature  of  the  insoluble 
constituent  (shellac  wax).  This  class  of  varnish  is  com- 
monly sold  and  used  in  its  turbid  condition,  the  insoluble 
constituent  indeed  being  an  intrinsic  and  necessary  part 
of  the  preparation  for  certain  purposes  (''French  polish- 
ing"). The  properties  and  uses  of  the  spirit  varnishes 
are  best  considered  by  reference  to  the  individual  resins 
which  have  been  described  in  Part  III.  Additional  facts  of 
technical  importance  may  be  given  under  the  following 
special  headings  of  the  spirit  varnish  resins. 

Shellac. — ^This  represents  the  most  important  constituent 
of  spirit  varnishes,  possessing  a  degree  of  elasticity  and 


SPIRIT  VARNISHES 


hardness  distinguishing  it  from  all  other  gums  and  resins. 
Shellac,  as  it  comes  into  the  market  in  its  various  forms  of 
flake  shellac,  button  lac,  garnet  lac,  etc.,  is  a  manufactured 
product,  the  different  treatments  producing  the  lac  in  various 
forms  and  conditions  of  purity. 

The  preparation  of  shellac  for  the  market  from  stick-lac 
embodies  the  separation  of  bark,  wood,  etc.,  removal  of 
colouring  matter,  and  conversion  of  the  resin  into  shellac. 
The  separated  colouring  matter  finds  an  outlet  on  the  market 
as  lac-dye.  Shellac  is  prepared  in  India  by  somewhat 
primitive  methods,  the  first  operation  of  separating  the  wood 
from  the  resin  being  done  by  crushing  the  stick-lac  by  hand, 
a  knife  being  used  to  assist  the  operation  of  separation.  The 
dye  is  removed  by  stirring  the  partly  purified  lac  in  a  closed 
cylinder  fitted  with  agitating  gear  with  water  or  a  weak 
solution  of  caustic  soda,  a  fair  proportion  of  the  dye  going 
into  solution  in  the  water.  The  water  is  decanted  off  and 
the  dye  may  be  precipitated  with  a  solution  of  alum.  The 
residue  is  then  carefully  dried,  transferred  to  a  roll  of  cotton, 
and  the  roll  containing  the  stick-lac  gradually  warmed  by 
bringing  it  into  proximity  of  a  fire  ;  the  ends  of  the  roll  are 
twisted  to  squeeze  the  molten  shellac  through  the  cotton. 
The  molten  lac  is  then  allowed  to  solidify  on  plates  or  water- 
cooled  rolls  from  which  it  is  removed  in  flakes  of  an  orange- 
brown  colour  constituting  the  flake  shellac  of  commerce. 

Button  lac  is  obtained  in  the  form  of  large  flat  or  slightly 
hollowed  buttons  by  stirring  the  crude  lac  with  hot  water 
for  several  hours  to  remove  the  colouring  matter,  melting 
and  casting  into  buttons.  Adulteration  of  this  variety  with 
rosin  is  not  uncommon,  such  admixture  made  partly  to 
facilitate  melting  during  manufacture. 

Garnet  lac  is  a  special  preparation  of  shellac  characterized 
by  being  free  from  contained  wax.  The  waxy  constituent 
of  shellac  being  insoluble  in  alcohol  a  clear  solution  is  not 
obtainable  from  any  variety  but  garnet  lac.  Shellac  freed 
from  wax  is  obtained  by  either  fractionally  precipitating  the 
wax  from  a  strong  alcoholic  solution  of  shellac  by  water,  the 
wax  separating  out  first  and  the  shellac  remaining  in  solution 


2i6   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


being  recovered  by  distillation  of  the  alcohol,  or  by  treating 
with  aqueous  solutions  of  alkali  carbonate  wherein  the 
shellac  dissolves,  leaving  the  wax  insoluble  as  a  scum.  In 
the  latter  method  the  shellac  is  recovered  by  acidification  of 
the  alkaline  solution. 

In  connection  with  bleached  shellac  it  must  be  pointed 
out  that  many  of  the  specially  desirable  properties  of  the 
original  shellac  are  lost,  among  them  being  the  elasticity 
and  ready  solubility  in  alcohol.  It  was  pointed  out  in  a 
former  chapter  that  the  determination  of  the  iodine  value 
of  shellac  was  a  method  for  estimating  the  colophonium 
content,  but  the  method  is  not  applicable  to  the  examination 
of  the  residue  from  a  solution  of  shellac  since  any  rosin 
present  in  the  resin  would  become  oxidized  during  evapora- 
tion of  the  solvent  with  consequent  loss  of  iodine  value. 
In  such  cases  a  quantity  of  shellac  varnish  representing  about 
2  grams  of  dry  shellac  is  weighed  into  a  separating  funnel, 
200  c.c.  of  petroleum  ether  (B.P.  35°-6o''  C.)  added,  followed 
by  100  c.c.  water  added  drop  by  drop  with  shaking.  This 
will  result  in  the  separation  of  two  layers  with  an  intermediate 
layer  of  shellac,  the  rosin  remaining  in  the  clear  petroleum 
ether  layer.  The  rosin  can  be  estimated  by  the  usual 
methods  (Mcllhiney,  /.  Am.  Chem.  Soc,  1908,50,  867). 

Of  the  various  designations  of  alcoholic  solutions  of  shellac 
may  be  cited  French  polish,''  patent  knotting,''  negative 
varnish,  bookbinders'  varnish,  label  varnish,  whilst  many 
other  varieties  are  obtained  by  suitably  modifying  the 
properties  of  the  resin  by  addition  of  other  resins,  i.e,  elemi, 
Venice  turpentine,  sandarac,  etc. 

The  remaining  resins  which  are  used  in  the  manufacture 
of  spirit  varnishes  are  entirely  of  vegetable  origin,  being 
solidified  balsams  exuding  from  different  species  of  trees. 
These  resins  include  Manila  copal  (soft  variety),  elemi, 
sandarac,  etc.  Of  these,  soft  Manila  copal  is  by  far  the  most 
widely  used  as  it  enters  into  the  composition  of  the  pale 
spirit  varnishes  where  high  gloss  forms  a  more  important 
consideration  than  elasticity.  The  occurrence  and  mode  of 
production  of  the  resin  has  been  described  in  Part  III.  It 


SPIRIT  VARNISHES 


217 


appears  in  the  market  as  pale  yellow  tears  or  lumps,  some 
varieties  of  which  agglomerate  together  by  slight  heat  and 
pressure  blocky  gum'').  The  degree  of  solubility  of 
different  samples  of  Manila  gum  varies  somewhat,  many  not 
being  soluble  in  spirit  and  leaving  a  stringy  residue.  Manila 
resin  is  principally  used  in  that  variety  of  varnish  known  as 
white  hard  spirit  varnish.'' 

Manila  varnishes  are  also  considerably  used  as  vehicles 
in  the  manufacture  of  spirit  paints  and  enamels.  Addition 
of  small  percentages  of  castor  oil  or  linseed  oil  fatty  acids 
is  often  made  to  reduce  the  brittleness  of  the  dry  film  of 
resin.  Sandarac,  as  one  of  the  hardest  of  the  spirit  soluble 
gums,  is  employed  as  an  addition  to  spirit  varnishes  to 
impart  hardness  to  the  film.  Its  brittleness  precludes  its 
extended  use  as  the  sole  constituent  of  a  spirit  varnish. 

Benzoin,  often  erroneously  termed  gum  Benjamin," 
is  the  product  of  Benzoin  officinale  (cf.  Part  III.).  It  is 
relatively  unimportant,  being  used  by  the  varnish  maker 
mostly  on  account  of  its  characteristic  odour.  The  alco- 
holic solution  of  gum  benzoin  appears  in  the  Pharmacopoeia 
as  tincture  of  benzoin  or  Friar's  Balsam.  Benzoin  finds  its 
principal  application  as  an  addition  to  spirit  varnishes  in 
small  proportion,  its  aromatic  odour  rendering  it  useful 
in  improving  the  odour  of  spirit  varnishes  intended  as 
leather  dressings. 

Acroides  Spirit  Varnishes. — ^The  red  acaroid  resin 
is  used  only  for  coloured  spirit  varnishes.  It  is  strongly 
light  absorbing,  and  a  concentrated  solution  containing  a 
little  castor  oil  or  copaiba  balsam  gives  a  red-coloured  film 
which  may  be  used  to  coat  the  windows  of  photographic 
laboratories  so  as  to  exclude  actinic  rays. 

It  is  often  mixed  with  shellac  spirit  varnishes  and  used 
as  shellac  substitute  as  it  dries  quickly  and  hard.  In 
North  America  the  red  resin  is  used  in  large  quantities  in 
leather  making.  Its  special  important  property  of  holding, 
even  in  spirit  solution,  small  quantities  of  rubber  enables 
it  to  furnish  elastic  films  which  are  stated  to  be  water- 
resisting.    It  is  used  in  sealing  wax  and  for  making  aromatic 


2i8    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


soaps.  Attempts  to  bleach  the  resin  have  not  been  suc- 
cessful and  the  strong  colour  of  both  varieties  limits  their 
use  except  for  knotting/'  The  high  percentage  of  insoluble 
matter  makes  purification  advisable  before  it  can  compete 
successfully  with  other  spirit  soluble  resins. 


Section  IV. -CELLULOSE  ESTER  VARNISHES 


A1.THOUGH  solutions  of  cellulose  nitrate  have  been  known 
and  used  for  a  number  of  years,  the  particular  properties 
shown  by  films  consisting  of  cellulose  acetate  tempered  by 
various  plasticizing  agents  have  brought  these  products  to 
the  forefront  during  the  War. 

The  cellulose  nitrate  (erroneously  referred  to  as  nitro- 
cellulose) varnishes  have  for  their  commonest  representatives 
the  well-known  collodions  and  celluloid. 

The  collodion  of  the  British  Pharmacopoeia  consists  of 
pyroxylin  or  cellulose  nitrate  Ci8H2i06(OH)  (NO 3)8,  dissolved 
in  a  mixture  of  three  parts  of  ether  and  one  of  alcohol.* 
The  resulting  emulsoid  appears  as  a  viscous  translucent 
colourless  liquid.  On  evaporation  it  leaves  a  harsh  brittle 
film  which  often  dries  porous  and  opaque,  owing  to  the 
presence  of  water  deposited  within  the  film  and  originating 
from  condensed  moisture  from  the  surrounding  air,  the  latent 
heat  of  evaporation  of  the  volatile  ether  solvent  causing 
considerable  lowering  of  temperature.  The  brittleness  of 
collodion  is  modified  by  addition  of  castor  oil  and  Canada 
balsam  in  flexible  collodion  or  collodion  flexile  (B.P.). 
The  high  rate  of  evaporation,  inflammability,  and  tendency 
to  dr}^  to  an  opaque  film  are  modified  in  commercial  forms 
of  cellulose  nitrate  solutions  by  replacing  the  ether-alcohol 
by  amyl  acetate  with  addition  of  castor  oil  to  impart  flexi- 
bility to  the  film.  I^ower  nitrates  of  cellulose  down  to  the 
dinitrocellulose  are  also  employed  as  a  basis  of  the  celMose 
nitrate  varnishes  in  combination  with  other  solvents  and 
non-solvents,  e.g,  acetone,  methylated  spirit,  benzol,  and 
petrol,  etc. 

*  Cellulose  trinitrate  (gun  cotton)  is  insoluble  in  a  mixture  of  ether 
and  alcohol. 

219 


220    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 

A  solution  of  nitrated  cotton  in  the  above-mentioned 
solvents  is  known  as  Zapon/'  a  colourless  lacquer  for 
protection  of  bright  metal  parts  from  rusting  and  tarnishing. 
As  an  adhesive  it  is  often  blended  with  shellac  to  give  a 
lustrous  coating  for  metals. 

Celluloid  Varnishes, — ^If  collodion  wool  is  mixed  with 
camphor  and  heated  under  pressure  it  is  transformed  into 
celluloid.  Celluloid  requires  the  same  solvents  as  collodion, 
giving  a  varnish  which  is  colourless,  lustrous,  and  adherent, 
which  does  not  crack  and  may  be  easily  coloured  like  all 
spirit  varnishes  by  solutions  of  aniline  dyes.  The  addition 
of  castor  oil  renders  the  coating  more  elastic  and  more 
adherent.  They  are  tougher  than  collodion  varnishes  and 
may  be  applied  in  thicker  coats,  e.g.  up  to  ^  in.  thick,  and 
the  coats  are  brilliant  and  adherent,  waterproof  but  highly 
inflammable. 

Cellulose  Acetate  Varnishes. — ^It  is  safe  to  assume 
that  the  great  importance  which  the  manufacture  of  this 
product  has  assumed  has  been  entirely  due  to  the  part 
which  cellulose  acetate  varnishes  dopes  have  played  in 
the  War.  Cellulose  acetate  films  have  the  distinctive 
property  of  possessing  relative  uninflammability,  in  addition 
to  which  those  obtained  from  cellulose  acetate,  prepared  under 
certain  specific  conditions,  when  applied  to  linen  fabric  have 
the  property  of  inducing  increased  tautness.  The  import- 
ance of  this  latter  property  will  be  recognized  when  considered 
from  the  standpoint  of  application  to  a  stretched  fabric, 
viz.  the  wings  of  aeroplanes.  The  solubility  "  of  cellulose 
acetates  in  different  media  is  dependent  on  the  mode  of 
preparation  of  the  ester  just  as  in  the  case  of  the  cellulose 
nitrates,  but  the  variety  chosen  for  the  preparation  of  air- 
craft dopes  is  not  dissolved  in  the  usual  nitrocellulose 
solvents,''  acetone  and  its  homologues  with  other  sol- 
vents "  and    non-solvents  "  being  usually  employed. 

In  the  manufacture  of  dopes,  plasticizing agents, 
e.g.  triphenyl  phosphate,  etc.,  are  added  to  reduce  the  brittle- 
ness  of  the  film  whilst  retardation  of  the  rate  of  evaporation 
of  the  volatile  continuous  phase  is  effected  by  addition  of 


CELLULOSE  ESTER  VARNISHES  221 


certain  liquids  of  high  boiling  point,  viz.  benzyl  alcohol, 
terpineol,  etc. 

The  lower  degree  of  inflammability  of  cellulose  acetate 
has  not  resulted  in  the  adoption  as  a  base  for  varnishes 
other  than  those  intended  for  covering  aircraft  wings  owing 
to  its  higher  cost  and  the  relative  unimportance  of  the 
inflammability. 

Cellulose  acetate  sols,  on  the  basis  of  a  different  variety 
of  ester  to  that  used  in  dopes,  are  used  in  the  manufacture  of 
artificial  silk  fibres.  It  must  be  mentioned  that  in  conse- 
quence of  the  high  viscosity  simultaneous  with  low  concentra- 
tion of  the  sols  of  both  cellulose  nitrate  and  acetate  their 
employment  in  place  of  either  copal  or  spirit  varnishes  is 
hardly  likely  to  take  place  when  the  question  of  the  necessity 
of  application  of  films  of  tolerable  thickness  comes  into 
question.  For  outside  wear  the  dope  is  sensitive  to  the  actions 
of  ultraviolet  rays  and  must  be  protected  by  incorporation 
with  a  protective  pigment,  moreover  its  water-resisting 
power  is  not  very  great  and  it  has  to  be  covered  with  a  varnish 
of  low  water-absorbing  power,  preferably  a  China  wood  oil 
varnish.  (For  an  account  of  the  commercial  development 
of  the  manufacture  of  cellulose  acetate,  see  E.  C.  Worden, 
J.  5.  C.      1919,  jc?,  389.) 


Section  V.-ANALYSIS  OF  VARNISHES 

The  usual  varnish  examination  includes  the  determination 
of  specific  gravity,  flash  point,  colour,  viscosity,  the  amount 
and  nature  of  the  solvent  or  thinning  spirit,  a  qualitative 
test  for  rosin,  and  the  gravimetric  estimation  of  the  ash 
yielded  on  ignition  of  the  varnish.  Moreover,  a  drying  test 
on  glass  or  on  a  prepared  wood  panel  and  examination  of 
the  rubbing  properties  as  well  as  the  action  of  hot  and  cold 
water  on  the  film  are  noted.  It  is  evident  that  the  above 
tests  give  inadequate  information  as  to  the  durability  of 
the  coating  which  can  only  be  satisfactorily  determined  by 
several  months'  exposure.  To  a  very  limited  degree  the 
expert  can  predict  the  properties  of  a  varnish  film,  but  the 
dependence  of  the  properties  on  composition  is  by  no  means 
certain,  owing  to  the  difficulty  in  estimating  the  proportions 
of  resin  and  oil  in  any  mixing  as  well  as  the  lack  of  know- 
ledge of  the  connection  of  the  properties  of  the  fused 
resins  with  their  physical  and  chemical  constants.  The 
separation  and  determination  of  the  thinner  is  comparatively 
easy  and  accurate  (de  Waele  and  Smith,  Analyst,  1918, 
^j,  408).  The  separation  of  the  resin  and  oil  is  difficult  and 
the  identification  of  the  components  is  by  no  means  certain. 
The  different  processes  employed  for  the  depolymerization 
of  the  gum  introduce  difficulties  in  the  identification  of  the 
separated  resins.  At  present  there  is  no  agreement  as  to 
the  estimation  of  the  component  oils  which  may  be  present 
in  a  more  or  less  polymerized  state.  The  changes  which 
occur  in  the  ageing  "  process  undoubtedly  modify  the 
properties  of  resin  and  oil.  A  summary  of  the  methods 
employed  in  varnish  examination  is  given  by  Boughtoa 

222 


ANALYSIS  OF  VARNISHES 


(Technological  Papers  of  the  Bureau  of  Standards,  No.  65  : 
Determination  of  Oil  and  Resin  in  Varnishes/'  Washington, 
1916).  It  is  evident  that  if  the  estimation  of  the  glyceryl 
radicle  in  an  oil  mixing  were  accurate  the  determination  of  the 
oil  would  be  easy,  although  no  distinction  could  be  drawn 
between  simple  and  polymerized  oil.  It  must  be  pointed 
out  that  the  saponification  value  of  a  varnish  is  useless, 
because  the  resins  contain  esters.  Attempts  to  separate 
the  resin  from  the  oil  by  use  of  special  solvents  has  not  met 
with  success.  Vorhees  {Bull.  109,  Bureau  of  Chemistry, 
Dept.  of  Agr.,  U.S.A.)  proposes  to  separate  resins  by  petro- 
leum ether,  which  dissolves  the  oil  and  part  of  the  resins, 
and  transforming  the  oils  into  linoxyn  by  oxidation.  Treat- 
ment with  chloroform  will  separate  the  resin  from  the 
insoluble  linoxyn.  Boughton  considers  the  method  in- 
accurate for  general  use.  The  separation  of  resins  and  oils 
has  been  attempted  by  an  esterification  method  depending 
on  the  more  rapid  esterification  of  the  oil  acids  in  the 
presence  of  resin  acids.  This  method  was  suggested  by 
Twitchell  (/.  Soc.  Chem,  Ind.,  1891,  10,  884),  and  has  been 
criticized  and  modified  by  many  workers  (Gill,  /.  Amer. 
Chem.  Soc,  1906,  1723;  Holley,  "Analysis  of  Paint  and 
Varnish  Products,''  1912,  259 ;  I^ewkowitsch, Oils,  Fats,  and 
Waxes,"  5th  Edition,  in,  165  (1915)  ;  Boughton,  loc.  cit.). 

In  the  opinion  of  one  of  the  writers  the  difficulty  lies  in 
the  proper  control  of  the  rate  of  esterification  of  the  oil  and 
resin  acids  as  will  be  evident  from  the  description  of  the 
process  :  after  the  removal  of  the  thinners  the  resins  and 
oils  are  saponified  and  the  unsaponifiable  matter  is  collected 
and  taken  as  part  of  the  resin.  The  soap  solution  is  treated 
with  acid  and  the  liberated  fatty  and  resin  acids  are  esterified 
in  absolute  alcohol  solution  by  gaseous  hydrochloric  acid 
whereby  the  former  are  transformed  into  ethyl  esters  and 
the  latter  are  considered  to  be  unattacked.  The  mixture 
of  oil  esters  and  resin  acids  is  treated  with  petroleum  ether 
and  shaken  with  dilute  aqueous  potassium  hydrate  which 
extracts  the  resin  acids.  The  petroleum  ether  contains 
the  oil  esters  which  are  weighed  directly.    From  the  aqueous 


224    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


alkaline  solution  addition  of  acid  liberates  the  free  acids, 
which  can  be  collected  and  weighed  as  resin  together  with 
any  solid  matter  which  is  insoluble  in  petroleum  ether.  If 
the  esterification  has  been  incomplete  the  resin  values  will 
be  high  owing  to  contaminated  oil,  whereas  if  it  has  been 
carried  too  far  the  oil  content  will  be  too  high  owing  to  the 
presence  of  resin  esters.  The  method  works  fairly  satis- 
factorily if  rosin  and  linseed  oil  are  alone  present,  but  with 
copal  resins,  polymerized  oil  or  China  wood  oil  mixings  the 
method  is  not  so  reliable.  The  following  figures  obtained 
by  one  of  the  writers  will  give  a  rough  idea  of  the  degree  of 
accuracy  of  the  method  if  the  esterification  is  carefully 
controlled  : — 


Found.* 

Gale. 

Found.* 

Calc. 

Turpentine 

.  41-36 

41-3 

40*45 

43*5 

Oil  (linseed) 

'  3877 

34-81 

42-29 

40-52 

Resin 

.  13-8 

14*0 

1477 

14-9 

Residue  (driers)  . , 

•      3  07 

1-89 

0-83 

1-34 

Mcllheney  {Proc.  Am.  Soc.  for  Test.  Mat.,  1908,  S,  596) 
has  modified  the  method,  but  Darner  {N.  Dak.  Agr.  Exp. 
Sta.  Paint  Bulletin,  1915,  i,  No.  6)  and  Boughton  [loc  cit.) 
find  that  the  modification  is  unsatisfactory  when  tung  oil 
is  present  and  the  separation  of  the  precipitated  resin  is 
very  troublesome.  The  results  obtained  by  one  of  the 
authors  is  in  agreement  with  Darner  and  Boughton  (see 
p.  225),  and  he  is  of  the  opinion  that  the  substitution  of 
methylated  ether  for  petroleum  ether  by  Boughton,  although 
preventing  emulsions  and  shortening  the  method,  introduces 
an  error  owing  to  the  partial  solubility  of  the  resin  acid  salt 
in  methyl  ether,  but  he  agrees  with  Boughton  that  methods 
involving  esterification  by  Twitchell  or  Wolff  Methods 
{Chem.  Zeit.,  1914,  jS,  369)  are  the  best  so  far  devised  for 
practical  use,  although  the  accuracy  leaves  much  to  be 
desired.  The  method  is  tedious,  and  to  obtain  satisfactory 
results  large  quantities  of  varnish  (60  grams)  ought  to  be 
used. 

An  attempt  to  estimate  the  amount  of  polymerized 
oil    was   made   by   treating   the   resin    and    oil  after 
*  Mean  of  3  analyses. 


ANALYSIS  OF  VARNISHES 


225 


Mcllheney's 
method. 

Resin      . .        . .        . .  20*5 

Oil  (linseed  and  tung  oil)  31*28 


Twitchell 
method. 
17-0 

34*5-36'4 


Calc. 
15-0 

35-87 


Scheme  of  Boughton's  Method. 


Varnish 

.  I 

Ignition 


Ash<%(A) 


Varnish 

I 

Heat  5  hours 
at  115**  C. 


Varnish 

Treatment  witL  alcoholic  KOH 
and  extraction  with  ether 


Non-volatile 
matter  %(B) 


I 


Part  of 
unsaponifiable 
matter  (X) 


Soaps  of  resins  and  fatty 
acids  and  part  of 
unsaponifiable  matter 


Treatment  with  HCl,  separation 
of  fatty  and  resin  acids  and  unsaponifi- 
able matter,  esterification  with  alcohol 
and  H2SO4  and  treatment  with  KOHaq 


Ethyl  esters  of  fatty  acids  and 
part  of  unsaponifiable  matter 

Treatment  with  alcoholic  KOH 
and  ether 


Soaps  of  resin 
esters 

Treatment  with 
HCl  extraction 
with  ether 


Part  of  unsaponifi- 
able matter 


Total  unsaponifiable 
matter  %  (C) 


Soaps  of  fatty 
acids 

Treatment  with 
HCl,  extraction 
with  ether,  etc. 

I 


Resin  acids  % 
(E) 


Fatty  acids  % 


D  =  Oil%. 

B  -  (A  -f  D)  =  Resin  %. 

Products  marked  X  may  be  disregarded  unless  it  is  desired  to  check  the 
percentage  of  resin  as  follows : 

(C  -f  E)  X  1-07  =  Resin  (%). 

removal  of  the  thinners  with  acetone  (Morrell,  /.  5.  C.  /., 
j^,  1915),  and  the  soluble  and  insoluble  portions  v^ere 
estimated  separately  for  gum  and  oil  content.  For  example, 
in  the  analysis  on  p.  224,  the  ratio  of  thick  to  thin  linseed 
s.  15 


226   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


oil  was  found  to  be  i  :  2*5,  a  ratio  quite  difterent  to  the 
calculated  amount  of  thick  oil  present,  but  explicable  by 
the  polymerization  occurring  during  making  and  ageing 
of  the  varnish. 

The  identification  of  the  resins  and  oils  is  difficult,  and 
it  is  evident  that  the  resins  and  oils  separated  in  the  analysis 
will  not  be  very  closely  connected  in  their  constants  with 
those  of  its  components.  For  example,  referring  to  analysis 
already  quoted  : 

Original  Oil  Resin  Original 

oil.  separated.        separated.  resin. 

Iodine  value    . .        . .  185         92-106  85*7  109 

Saponification  value  ..  190         172-186      loi-iio  120-130 
Acid  value      ..  1-2  1*4  27  22-28 

The  presence  of  polymerized  oil  accounts  for  the  reduction 
in  the  iodine  value  as  well  as  absorption  of  HCl  during  the 
Twitchell  esterification.  Nevertheless  some  deductions 
may  be  drawn  as  to  distinction  between  soft  Manilla  and 
Congo  copals. 

The  great  difficulty  in  the  estimation  of  a  varnish  lies 
in  the  necessity  of  using  large  quantities  of  material  and  the 
tediousness  of  the  separation.  In  fact,  the  analysis  of  a 
varnish  constitutes  a  research  which  makes  it  difficult  to 
obtain  results  in  reasonable  time  for  practical  purposes. 
There  is  great  need  for  a  rapid  and  accurate  method  of 
examination.  For  the  estimation  of  metallic  driers  the 
ignition  of  the  varnish  and  estimation  of  the  metallic  oxides 
by  the  usual  gravimetric  or  volumetric  methods  is  quite 
straightforward. 

A.  de  Waele  {Proc.  Oil  and  Colour  Chemists'  Assoc.,  1920, 
77,  75)  draws  attention  to  the  necessity  of  interpretation  by 
a  varnish  technologist  of  the  restilts  obtained  by  analj^sis, 
and  on  a  basis  of  consideration  of  the  technology  of  the 
varnish-making  process  gives  a  method  of  which  the  follow- 
ing is  a  resume.  The  method  is  somewhat  involved  in 
manipulative  detail,  and  the  reader  is  referred  to  the  original 
paper. 

The  volatile  thinner  is  estimated  by  a  simplified  method 
of  distillation  with  water  (A.  de  Waele  and  F.  Smith,  Analyst, 


ANALYSIS  OF  VARNISHES 


1917,  ^2,  170,  and  A.  de  Waele,  Analyst,  1918,  ^j,  408), 
two  to  three  grams  of  varnish  only  being  used  for  the  deter- 
mination, and  the  residue  of  fixed  oils,  resin,  and  drier  being 
employed  for  the  remainder  of  the  analysis.  The  non- 
volatile residue  is  saponified  with  strong  potassium  h3^droxide 
in  benzol-alcohol,  the  soap  evaporated  to  dryness,  taken  up 
with  water,  acidified  under  ether,  and  the  aqueous  layer, 
consisting  of  the  mineral  matter  present  as  drier,  etc., 
separated  off  and  discarded.  The  acid  ethereal  layer  is  then 
shaken  up  with  excess  of  potash  and  the  unsaponifiable 
matter  remaining  in  the  ethereal  layer  estimated  in  the  usual 
way.  The  intermediate  step  of  acidification  of  the  soap 
solution  with  subsequent  reconversion  into  soap  has  for  its 
object  the  removal  of  lead,  calcium,  etc.,  which  might  be 
present  from  rosinate  and  which  would  subsequently  interfere 
with  convenient  separation. 

The  soap  freed  from  unsaponifiable  matter  is  then  acidified 
under  petroleum-ether  (b.p.  35*^-60°  C),  whereby  ''gum- 
resin  acids  separate  as  a  flocculent  intermediate  layer. 
These  are  removed,  dissolved  in  ether,  and  weighed.  The 
petroleum-ether  layer,  consisting  of  resin  acids  from  both 
rosin  and  gum-resin  together  with  fatty  acids  from  the  oil  are 
then  evaporated  to  dryness  and  esterified  in  95  per  cent, 
alcohol  with  i  c.c.  of  strong  hydrochloric  acid  as  catalyst 
for  30  minutes  at  boiling  temperature.  An  equal  volume 
of  petroleum-ether  is  added,  the  acid  present  neutralized 
with  normal  alkali,  and  water  added  to  an  amount  sufficient 
to  dilute  the  alcohol  present  to  60  per  cent,  strength.  This 
ensures  adequate  separation  of  the  alcohol  and  petroleum- 
ether  layers  whilst  inhibiting  hydrolysis  of  the  rosin  soap. 
The  aqueous  layer  is  then  separated,  the  petroleum-ether 
layer  re-extracted  with  water  once  or  twice  and  the  extracts 
united.  The  aqueous  extract  is  finally  acidified  under 
ether,  which  latter  will  then  contain  the  resin  acids  from 
rosin  and  part  of  the  gum-resin. 

For  the  calculation  of  analytical  results,  the  original 
paper  must  be  consulted,  as  the  author  makes  provision  for 
the  cases  of  differently  constituted  varnishes.    In  brief, 


228    RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


however,  the  gum-resin  present  is  obtained  from  the  un- 
saponifiable  matter  (less  a  deduction  for  unsaponifiable  matter 
from  the  oil  present),  together  with  the  gum-resin  acids,  a 
factor  being  employed  on  account  of  undeterminable  fraction 
lost  as  water  soluble,  etc.  Rosin  is  obtained  from  the  unsa- 
ponifiable matter  and  resin  acids,  the  allocation  of  these  two 
fractions  to  rosin  being  determined  by  special  considerations 
of  composition. 

By  a  series  of  tests  on  the  petroleum-ether  insoluble 
layer  the  author  differentiates  between  true  gum  acids," 
oxidized  fatty  acids,  and  oxyabietic  acid  from  rosin.  By 
the  recognition  of  a  fraction  of  the  rosin  occurring  as 
petroleum-ether  insoluble,  the  author  probably  accounts 
and  corrects  for  the  low  figures  obtained  for  rosin  in  Mcll- 
heney's  method  {Proc,  Amer,  Soc.  for  Test,  Mat.,  8,  596). 

The  method  of  analysis  is  supplemented  by  a  series  of 
physical  tests,  amongst  which  it  is  interesting  to  note  that 
the  author  differentiates  between  physical  and  chemical 
weather  resistance.''  Physical  weather  resistance  is  re- 
ferred to  as  the  provision  of  adequate  elasticity  and  hardness 
unaer  the  particular  conditions  of  exposure  of  the  product, 
and  is  thus  dependent  on  ratio  of  resin  to  oil,  whilst  chemical 
weather  resistance  is  determined  by  the  stability  of  the  oil 
component  to  atmospheric  oxidation  beyond  that  limit 
which  marks  the  maximum  increase  in  weight  on  oxidation 
(see  p.  41).  For  the  determination  of  the  latter,  the 
author  allows  20  volume  "  hydrogen  peroxide  to  evaporate 
on  the  dried  film  of  varnish,  and  notes  the  relative  corrosions 
effected  by  this  reagent.  The  paper  embodies  also  standard- 
ized procedures  for  determining  resistance  to  water  (''  chalk- 
ing etc.,  and  includes  suggested  specifications  for  various 
classes  of  varnishes. 


BIBLIOGRAPHY 


Part  II.— DRYING  OILS. 

Andes,  L.  E.,  "  Drying  Oils,  Boiled  Oil,  and  Solid  and  Liquid  Driers," 
2nd  edition,  revised  by  H.  B.  Stocks.  1920. 

Chalmers,  T.  W.,  "  Production  and  Treatment  of  Vegetable  Oils." 
1919. 

Engler,  C,  and  Weissberg,  J.,  "  Kritische  Studien  in  die  Vorgange  der 
Autoxidatipn."  1904. 

Fahrion,  W.,  "  Die  Chemie  der  trockenden  Oele."    191 1. 

Fryer,  P.,  and  Weston,  F.  W.,  "  Technical  Handbook  of  Oils,  Fats,  and 
Waxes,"  2  vols.    191 7. 

Henderson,  G.  G.,  "  Catalysis  in  Industrial  Chemistry."    191 9. 

Ingle,  H.,  "  A  Manual  of  Oils,  Resins,  and  Paints,"    Vol.  i.  1915. 

Lewkowitsch,  J.,  "  Chemical  Technology  and  Analysis  of  Oils,  Fats,  and 
Waxes,"  5th  edition,  3  vols.  1915. 

Livache,  Ach.,  and  Mcintosh,  J.  G.,  "  Manufacture  of  Varnishes  and 
Kindred  Industries."  Vol.  i.  "  The  Crushing,  Refining,  and  Boiling  of 
Linseed  Oil."  1920. 

Marcusson  J.,  "  Laboratoriumsbuch  fiir  die  Industrie  der  Oele  und 
Fette."    191 1. 

Newton,  A.  P.,  "  The  Staple  Trades  of  the  Empire."    191 7- 
Pickering,  G.  F.,  "  Aids  to  the  Commercial  Analysis  of  Oils  and  Fats." 
1917. 

Rideal,  E.,  and  Taylor,  H.,    Catalysis  in  Theory  and  Practice."  1919. 
Seeligman,  F.,  and  Zieke,  F.,  **Handbuch  der  Lack-  und  Firnis-In- 
dustrie."  1914. 

Ubbelohde,  L.,  "  Handbuch  der  Chemie  und  Technologic  der  Oele  und 
Fette."  1908. 

Wright,  A.  C,  "  Analysis  of  Oils  and  Allied  Substances."  1903- 

Part  HI.— RESINS  AND  PITCHES. 

Abraham,  H.,    Asphalts  and  Allied  Substances."    191 8. 
Bottler,  M.,  "  Harz  und  Harz  Industrie."  1907. 

Dieterich,  K.,  "  Analysis  of  Resins  and  Balsams,*'  revised  by  H.  B. 
Stocks,  2nd  English  edition.  1920. 

Livache,  Ach.,  and  Mcintosh,  J.  G.,  ''Manufacture  of  Varnishes  and 
Kindred  Industries,"    Vols.  2  and  3.  1908. 

Lunge,  G.,  and  Keane,  C.  A.,  Technical  Methods  of  Chemical  Analysis 
(Pitches)."    191 1. 

Schweizer,  V.,  "The  Distillation  of  Resins,"  translated  by  H.  B. 
Stocks,  2nd  English  edition.  1920. 

229  15* 


230   RUBBER,  RESINS,  PAINTS  AND  VARNISHES 


Seeligman,  F.,  and  Zieke,  F.,  "  Handbuch  der  Lack-  und  Firnis  -In- 
dustrie." 1914. 

Thorpe,  T.,  "  Dictionary  of  Applied  Chemistry  "  (Pitches).  1913. 

Tschirch,  A.,  "  Die  Harze  und  die  Harzbehalter."  1906. 

Wiesner,  "  Die  Rohstoffe  des  Pfianzenreiches."  Gummiarten  :  Harze. 

Part  IV.— PIGMENTS,  PAINTS,  AND  LINOLEUM. 

Andes,  L.  E.,  "  Iron  Corrosion,  Anti-fouUng  and  Anti -corrosive  Paints," 
2nd  edition.  1920,  "  Oils,  Colours,  and  Printing  Inks,*'  2nd  Enghsh 
edition.  1918. 

Church,  A.  H.,  "  The  Chemistry  of  Paints  and  Painting,"  3rd  edition. 
1907. 

Friend,  W.,  '*  An  Introduction  to  the  Chemistry  of  Paints."    191 1. 
Gardner,  H.  A.,  "  Paint  Researches  and  their  Practical  Application." 
1917. 

Gardner,  H.  A.,  and  Schaeffer,  "  Analysis  of  Paints."    191 1. 

Holley,  D.,  and  Ladd,  F.,  "  Paints  and  their  Composition."  1908. 

Hurst,  H.,  Painters*  Colours,  Oils,  and  Varnishes,"  2nd  edition.  191 3 . 
"  The  Painter's  Laboratory  Guide."  1901.  "A  Dictionary  of  Chemicals 
and  Raw  Products  used  in  Paints  and  Varnishes,"  2nd  edition.    191 9. 

Jennings,  A.  S.,  "  Paint  and  Colour  Mixing."  1906. 

Jennison,  F.  H.,  "  The  Manufacture  of  Lake  Pigments  from  Artificial 
Colours,"  2nd  edition.  1920. 

Jones,  M.  W.,  ''History  and  Manufacture  of  Floorcloth  and  Linoleum/* 
/.  5.C./.,j5,26t,  (1919). 

Petit,  "Manufacture  and  Comparative  Merits  of  Lead  and  Zinc  White 
Paints."    1 91 9. 

Scott,  W.  G.,  "  White  Paints  and  Painting  Materials."    191  o. 

Smith,  J.  Cruickshank,  Paints  and  Painting  Defects."  191 3.  *'The 
Manufacture  of  Paint,"  2nd  edition.    191 9. 

Toch,  Max,  "  Chemistry  and  Technology  of  Mixed  Paints,"  2nd  edition. 
1916. 

Part  V.— VARNISHES. 

Andes,  L.  E.,  Die  Fabrikation  der  Kopal  und  Spiritus  Lacke."  1909. 
"  Die  Surrogate  in  der  Lack,  Firnis  und  Farben  Industrie."  1908. 

Bottler,  M.,  Die  Lack  und  Firnis  Fabrication."  1908.  "  Harz  und 
Harz  Industrie."  1907. 

Coffignier,  C,  Manuel  du  Fabricant  des  Vernis,  Gommes,  Resines, 
T6rebenthines,  Vernis  Gras,  etc."  1908. 

Fleming,  A.  P.,  and  Johnson,  R.,  '*  Insulation  and  Design  of  Electric 
Windings."    191 3. 

Hurst,  H.,  Paints,  Colours,  Oils,  and  Varnishes,"  2nd  edition.  (Chapter 
on  Varnishes  by  Dr.  Blackler.)    191 3. 

Livache,  Ach..,  and  Mcintosh,  J.  G.,  "  Manufacture  of  Varnishes  and 
Kindred  Industries."    Vols.  2  and  3  (Spirit  Varnishes).  1908. 

Sabin,  H.,  The  Industrial  and  Artistic  Technology  of  Paint  and 
Varnish."  1905. 

Sabin,  H.,  and  Bottler,  M.,  "  German  Varnish  making,  with  notes  on 
American  Varnish  Manufacture."  1912. 

Seeligman,  F.,  and  Zieke,  F.,  Handbuch  der  Lack-  und  Firnis-In- 
dustrie."  1914. 


SUBJECT  INDEX 


Abietic  acid,  96 
Absorption  of  oxygen — 

by  linseed  oil,  41,  63,  68 

by    boiled,    blown,    and  poly- 
merized linseed  oil,  63 
Acaroides  spirit  varnish,  217 

resin,  101 
Acrolein,  41 
Alumina,  124 
Alizarine  lake,  128 
Angola  copal,  83 
Analysis  of  varnishes,  221 
Animi  copal,  83 
Asphaltum,  103 
Autoxidation,  67 

Balsams,  78 
Bakelite,  99,  211 
Barytes,  121 
Benzoin,  80,  217 

BibUography    of    rubber  hydro- 
carbons, 33 
of  drying  oils,  228 
resins,  228 

pigments  and  paints,  230 

varnishes,  230 
Bitumen,  103 

origin  of,  107 
Bituminous  varnishes,  204 
Blanc  fixe,  121 
Blown  oil,  60 
Boat  varnish,  204 
Body  varnish,  204 
Boiled  oil,  60 
Brazilian  copal,  85 
Brunswick  black,  205 
Button  lac,  215 

Canada  balsam,  78 
Carriage  varnish,  204 
Catalysis,  41,  70,  73 
Ceara  rubber  seeds,  29 
Celluloid,  220 
Cellulose  acetate,  220 
ester  varnishes,  219 
Cerium  tungate,  72 


Chalking  of  paints,  117 
Chinese  lacquer,  98 
China  clay,  124 
Cholesterol,  36 
wood  oil,  57 
Church  oak  varnish,  203 
Coachbuilders'  varnish,  203 
Cobalt  driers,  188 
Collodion,  219 

Colloidal  suspension  of  pigments, 
113 

Colophonium,  80 
Colophony,  95 
Colour  of  pigments,  109 
Congo  copal,  85 
Copals,  properties  of,  83 
Copaiva  balsam,  78 
Cork  carpet,  174 
Corticine  process,  174 
Cumarone  resin,  100 

Dambonite,  15 
Dammar,  87 

Decorative  varnishes,  203 
Development  of  rubber  planting,  2 
Dimethyl  fulvene,  38 
Dipentene,  19 
Diseases  of  rubber,  31 
Distempers,  159 
Driers,  theories  of,  67 
Drying  oils,  35-60 

bibhography,  229 

candlenut  oil,  60 

China  wood  oil,  57 

hemp-seed  oil,  60 

Japanese  wood  oil,  60 

linseed  oil,  35 

menhaden  oil,  60 

Niger,  60 

pararubber-seed  oil,  30,  56 
perilla  oil,  55 
poppy-seed  oil,  56 
soya  bean  oil,  55 
sunflower  oil,  60 
walnut  oil,  60 
methods  of  testing,  73 


231 


232 


SUBJECT  INDEX 


Ebonite,  26 
Elemi,  79 
Enamels,  137 

manufacture  of,  147 
Eosin  lakes,  129 
Erythrene,  33 
Ester  gums,  97 
Extenders,  121 

Feeding  of  paints,  119 
Fillers,  121 

Flatting  of  varnishes,  194 
Flax  plant,  49 
Floorcloth,  5 
French  oil  varnish,  204 

polish,  214 
Fiintumia  elastica,  30 

Garnet  lac,  215 
Goldsize,  193 
Green  earth,  135 
Gum  arable,  73 

tragacanth,  78 
Gum  running  pots,  185 
Gutta  percha,  27 

Hardening  of  oils,  40 
Hevea  Braziliensis,  1,  9 

Indian  ink,  113 
Indene  resins,  100 
Inside  varnish,  203 
Insulating  varnishes,  208 

classification,  207 

for  impregnating  windings,  207 

cementing,  210 

for  fabrics  and  papers,  209 

finishing,  210 
Iron  oxide  reds,  129 
Isoprene,  19,  31 

Japans,  204 

Kaolin,  124 
Kauri,  83 
Ki-urushi,  98 
Knotting,  216,  218 

Lacquer,  Chinese  and  Japanese, 
98 

Laevulinic  aldehyde,  1 8 

acid,  18 

peroxide,  18 
Lakes,  green,  135 

red  pigments,  127 
Latex,  function  of,  7 

tapping,  8,  12 

chemical  properties,  13 

physical  properties,  12 


Linoxyn,  composition,  38 

Linolenic  acid,  37 

Linoleum — ■ 

binding  medium,  165 
cement  pan,  170 
Corticine  process,  174 
granular  inlaying  process,  173 
machine  inlaying  process,  175 
oxidizing  shed,  166 
Walton's  shower  bath,  167 
smacker,  169 

Linolic  acid,  37 

Lithographic  varnishes,  60 

Lithol  red,  R,  128 

Lithopone,  118 

Livering  of  paints,  118 

Madder  lake,  128 
Manganese  driers,  188 
Manihot  glazovii,  10 
Manila  copal,  83,  216 

spirit  varnishes,  217 
Manufacture  of  paints  and  enamels, 
147 

Maple  varnish,  204 
Mastic,  88 
Mixing  varnish,  203 
Moisture,  action  on  glycerides,  41, 
48 

jS-Myrcene,  32 

Oil  absorption  of  pigments,  140 
Oleic  acid  ozonide,  39 
Opacity  of  pigments,  109-111 
Oxidation  of  linseed  oil,  influence  of 

moisture  on,  44 
Ozonides  of  rubber,  17 
Ozokerite,  105 

Paints,  137-162 

bibhography,  230 

black,  158 

commercial,  158 

green,  157 

manufacture,  147 

medium  of,  144 

mills,  148-151 

pigment  of,  146 

red  and  brown,  155 

volatile  thinners  of,  146 

water,  161 

white,  154 

yellow,  156 
Para  rubber,  9 
Peroxides,  37 
Phytosterol,  36 
Pigments — 

bibliography,  230 

chemical  effect  on  media,  111 


SUBJECT  INDEX 


233 


Pigments  (continued) — 

physical  effect  on  paint,  112 
black — ■ 

black  oxide  of  iron,  135 

bone  black,  136 

ivory  black,  136 

lamp  black,  136 

vegetable  black,  136 
blue — • 

lime,  ultramarine  and  Prussian, 
136 
brown — 

burnt  sienna,  132 

Cappagh  brown,  132 

Cassel  earth,  132 

raw  sienna,  131 

raw  and  burnt  umber,  132 

Vandyke,  132 
*  green — • 

Brunswick,  134 

chrome,  135 

emerald,  135 

Guignet's,  135 

purple,  132 
red — 

American  vermilion,  129 

Chinese,  129 

colcothar,  130 

Derby,  129 

iron  oxide  reds,  129 

Paranitr aniline  red,  128 

Persian,  129 

red  lakes,  127 

rouge,  130 

Turkey  red,  130 

Venetian  red,  130 

vermilion,  127 
white — • 

basic  lead  sulphate,  117 

lithopone,  118 

sublimed  white  lead,  117 

white  lead,  114 

zinc  white,  118 
yellow  and  orange — • 

barium  chromate,  125 

lead  chromates,  125 

ochres,  126 

orange  lead,  129 

zinc  chromate,  126 
Piperylene,  33 
Pitches,  101-107 
bibUography,  229 
bitumen,  103 
coal  tar,  102 
identification,  106 
petroleum,  103 
stearine,  103 
rosin,  105 
Plasticizing  agents,  220 
Polishing  varnishes,  204 


Polymerized  linseed  oil,  65,  224 
Preparation  varnish,  194 
Problems  of  protection  of  surfaces 

by  paints  and  enamels,  137- 

147 

Ready-mixed  paints,  137 
Rescues,  79 
Resins,  77 

acidity,  86 

bibliography,  229 

Canada  balsam,  92 

classification,  82 

colophony  (rosin),  90,  95 

copals,  83 

cumarone  and  indene,  100 

dammar,  87 

formation  in  plant,  81 

hardness,  81 

lac,  88 

mastic,  88 

solubility,  87 
Resinolic  acids,  79 
Resinols,  79 
Rosin  esters,  97 
Rosin  oil,  90,  97 
Rubber — 

bibliography,  33 

chemical  properties,  16 

composition,  16 

diseases  and  pests,  31 

origin  and  distribution,  5 

output,  11 

physical  properties,  1 6 

seeds  and  oils,  28 

substitutes,  28 

testing,  19 
Rubber  articles,  manufacture  of,. 21 

vulcanization,  23 
Rubber- bearing  species,  8 

Shellac,  214 

Silica,  124 

Spinacene,  36 

Spirit  varnish  resins,  87 

Spirit  varnishes,  213-218 

Staining  power  of  pigments,  109 

Stand  oil,  60 

Suspending  power  of  pigments,  113 
Synthetic  resins,  99 
rubber,  32 

Terpenes,  93 

Testing  of  drying  oils,  75 

Tintometer,  110 

Tinctorial  power  of  pigments,  109- 

111 
Tung  oil,  57 
Turpentine,  90 

substitutes,  94,  190 


234  NAME 

Ultraviolet  light,  action  on  linseed 
oil,  46 

Undercoating  varnish,  194 
Urushiol,  99 

Varnish — 

analysis,  222-228 
ageing  of,  189,  226 
constituents,  181 
driers,  187 
foots,  189 

manufacture,  183-189 

properties  on  application,  193 
Varnishes,  defects  of — 

bHstering,  202 

blooming,  198 

chalking,  198 

cissing,  197 

webbing,  201 

wrinkling,  200 
Varnishes  (oil),  179-205 
(tung  oil),  191 


NAME 


Abraham,  2  9 
Adriani,  13 
Albert,  100 
Allsebrook,  128 
Andes,  63,  229,  230 
Armitage,  57 
Armstrong,  94 

Barrowcliff,  35 

Bary,  16 

Bawtree,  111 

Bedford,  37,  40 

Behrend,  100 

Bischofi,  96 

Blanes,  96 

Bottler,  84,  100,  230 

Bouchardt,  31 

Boughton,  222 

Brimsdown  Lead  Co.,  115 

Brock,  43 

Butlerow,  6 

Caspari,  33 
Chalmers,  229 
Chapman,  36 
Christy,  33 


INDEX 

Varnishes  (continuedy-^ 

(spirit),  213-218 

bibliography,  230 
Varnishing,  198 
Vermilionettes,  129 
Vulcanization  of  rubber — 

hot,  23 

cold,  26 
Vulcanized  rubber  articles — 

belting,  26 

cut  sheet,  25 

elastic  thread,  25 

hose,  26 

tyres,  25 
Vulcanite,  26 

Water  paints,  161 
White  lead  "  in  oil,''  116 
Whiting,  123 

Zanzibar  copal,  85 


INDEX 


Church,  230 
Coffignier,  87,  230 
Collins,  1 
Couturier,  32 
Cross,  14 

Darner,  224 
De  Jong,  83 

De  Waele,  37,  38,  42,  45,  65,  139, 

140,  169,  226,  227 
Dieterich,  81,  229 
Ditmar,  33,  34 
Donath,  104 

Ellis,  97 

Engler,  37,  67,  70  229 
Erdmann,  37,  39 
Esch,  49 
Eyre,  49 

Fahrion,  37,  38,  59,  70.  96,  229 

FenaroH,  39 

Fleming,  206,  207,  230 

Fokin,  38,  44,  67 

Frankenstein,  57 

Friend,  36,  40,  42,  47,  55,  230 

Fryer,  40,  73,  75,  229 


NAME 


Gardner,  46,  47,  56.  66,  90,  106, 

107,  156,  230 
Genthe,  46,  67,  70 
Girard,  16 
Goldsobel,  37,  69 
Goodyear,  23 

Hancock,  1 

Harries,  4.  7,  16,  17,  34,  38 
Hazura,  67 
Heaton,  65 
Heaven,  37,  42 
Heil,  34 

Henderson,  70,  73,  229 
Henry,  86 
Herbst,  33 
Hinrichsen,  33 
Holley,  223,  230 
Hurst,  65,  116,  230 

Indestructible  Paint  Co.,  87 
Ingle,  37,  38,  43,  48,  64,  68,  69,  70, 
72,  74,  105,  229 

Jennings,  230 
Jennison,  230 
Johnson,  206,  207 
Jones,  230  ' 

Keane,  96.  107,  230 
Kissling,  67 
Klages,  34 
Kondakow,  34 
Krumbhaar,  100 

Ladd,  96,  230 
Langton,  104 
Lawrance,  11 
Lawton,  106 
Lebach,  99 
Leeds,  66 
Lewis,  73 
Lewis,  E.  W.,  33 

Lewkowitsch,  55,  66,  73,  223,  229 
Liddle,  41 
Lippert,  41,  67 

Livache,  72,  73,  81,  93,  96,  101,  229 
Lovibond,  110 
Lunge,  96,  107,  230 

Mackey,  48,  68 
Mcllhiney,  216,  224,  228 
Mcintosh,  81,  93,  95,  101,  229,  230 
Maitland,  A.  D.  Steel,  49 
Majima,  59,  99 
Manders,  33 
Mansbridge,  106 
Marcusson,  103,  105,  229 
Harden,  48 
Margosches,  104 


INDEX  235 

Mastbaum,  66 

Meguele,  97 
Melhuish,  56 
Mellor,  73 
MoUnari,  39 
Morgan,  33 

Morrell,  37,  38,  49,  68,  66,  71,  72, 

76,  200,  201 
Muehle,  96 

Nastvogel,  96 
Newton,  229 
Nicolet,  41 

Orloff,  38 
Orr,  118 

Ostromyslenski,  32,  34 

Parkes,  24 
Parnacott,  174 
Paul,  96,  181 
Pearson,  48 
Pearson,  H.  C,  34 
Peckham,  107 
Pfund,  109,  118 
Pelher,  33 

Perkin,  F.  M.,  92.  93 
Perkin,  W.  H.,  34 
Perrott,  137 
Pickering,  229 
Pickles,  16 
Potts,  33 
Power,  36 
Preyer,  16 
Probeck,  190 

Rabinovitz,  97 
Raspe,  37,  39 
Redman,  43 
Redwood,  106 
Reid,  64 

Richardson,  102,  107 
Rideal,  E.,  73,  229 
Rideal,  S.,  34 
Ridley,  1 
Rosicki,  97 

Rubber  Growers'  Association,  2 

Sabatier,  40 
Sabin,  230 
Salway,  38,  41 
Sawyer,  190 
Schaeffer,  107 
Schidrowitz,  33 
Schkateloff,  92 
Schulter,  106 
Schumann,  58 
Scott,  G.,  and  Sons,  64 
Scott,  W,  G.,  230 
Seaton,  190 


236  NAME 

Seeligman,  101,  107,  199,  211,  229, 
230 

Senderens,  40 
Smith,  226 
Steele,  75 
Stevens,  57 
Stewart,  102 
Strasser,  104 

Tahara,  99 

Taylor,  174 

Terrisse,  87 

Terry,  34 

Thiessen,  137 

Thorpe,  230 

Tilden,  32,  34 

Toch,  60,  66,  230 

Torrey,  33 

Tsujimoto,  36 

Tschirch,  79,  83,  91,  92,  230 

Twiss,  25 

Twitchell,  223,  226 

Ubbelohde,  229 

Vernet,  7,  8 
Vincent,  62 


INDEX 

Wallach,  93 
Walton,  163 
Washburn,  75,  95 
Weber,  C.  O.,  16,  33 
Weber,  40 
Weger,  67 
Weidert,  17 
Weissberg,  67 
Weith,  43 

Weston,  40,  56,  73,  76,  229 

Whitby,  53 

Whitfield,  16 

Whitley,  3 

Wickham,  1 

Wiesner,  80,  81,  230 

Williams,  211 

Williams,  Greville,  31 

Willstatter,  34 

Wilson,  37,  42 

Wohl,  100 

Wolff,  47,  201,  224 

Woodmansey,  64 

Worden,  221 

Wright,  A.  C,  66,  74,  229 
Wright,  H.,  33 

ZiEKE,  101,  107,  199,  211,  229,  230 


THE  END 


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